Author: Turambar (extracted from Symbolic Flight)

Ron/Hermione shippers believe that Hermione, despite alot of evidence to the contrary, has romantic feelings for Ron to compliment his crush on her. Harry/Ginny shippers believe that Ginny has unresolved romantic feelings for Harry and that Harry finds her attractive. Apparently Ron and Hermione are suited because they are opposites while Harry and Ginny are suited because they have personality and interests - bravery, recklessness, quidditch - in common.
Well let's imagine that R/Hr and H/G do get together, as unrealistic as that seems to me. What then? What does OOTP tell us about these personalities and how they would react?

HARRY:
Harry was very upset at R/Hr spending time together at the beginning of OOTP without him.
There were other factors at work here - an overall anger at being kept in the dark, being sidelined despite his experience, being left at the Dursleys - but the personal hurt was strong and so was the impression he didn't understand quite what he's feeling.
"But when were they going to see him?...but how soon was soon...He could hardly bear to think of the pair of them having fun at The Burrow when he was stuck in Privet Drive...In fact, he was so angry with them he had thrown away, unopened, [birthday presents]....Why had Ron and Hermione got together without inviting him along too?...All of a sudden after yearning to see them for a solid month - he felt he would rather Ron and Hermione left him alone....'So how come I have to stay at the Dursleys' while you two get to join in everything that's going on here...So you haven't been in the meetings, big deal. You've still been here, haven't you? You've still been together'...Every bitter and resentful thought Harry had had in the past month was pouring out of him: his frustration at the lack of news, the hurt that they had all been together without him...'I suppose you've been having a real laugh, haven't you, all holed up here together....So what have you two been doing, if you're not allowed in meetings?' he demanded. 'You said you'd been busy."

Also all three felt very awkward about R/Hr having to sit without Harry in the prefects' carriage on the train journey.
The interesting thing is that R/Hr have only had very small periods of time together: meeting up a bit before Harry in POA and GOF, small periods of time at school while Harry has played quidditch. Remember his reaction to their trip to Hogsmeade in POA where, feeling down because he was missing out, he felt they looked as though they'd had the 'time of their lives' when they came back.

In contrast Harry is used to Hermione spending time with him without Ron. This began in PS with the Norbert adventure while Ron was in the hospital wing and later Hermione going on with him for the Stone after Ron is knocked out rather than staying behind to get help for Ron. In POA Ron is dragged away by Sirius and H/Hr go to the Shrieking Shack together, Ron is knocked out again and in hospital again while H/Hr undertake the time-turner adventure. In GOF Hermione chooses to spend most of her time with Harry during his fight with Ron before the First Task, including spending hours helping him train, on a visit to Hogsmeade where she sees Ron but doesn't speak to him and sitting with him in class. In OOTP H/Hr have two forest adventures together without Ron.

Harry agrees to go with Hagrid while Ron is playing in the quidditch final on his and Hermione's behalf, without Hermione verbally agreeing. He expected her, based on past behaviour, to go with him to help Hagrid rather than stay and watch Ron on his big day.
Imagine if that pattern of behaviour had been different: and JKR had written the books to emphasise a deepening friendship between R/Hr. For instance if she had written when Ron got knocked out in PS that Hermione ran over to Ron's side and said to Harry 'I'll go and get help for Ron', or that it was Ron who pulled Millicent off Hermione in COS, or that Hermione ran towards both boys yelling 'Harry, Ron, I knew you'd work it out' in COS, or that it was Ron's arm she grabbed for support when the Fat Lady was slashed in POA, or that Hermione rushed over to Ron looking shaken and upset and tried to comfort him when he broke his leg in POA or that Hermione ran to get help when Ron was knocked out in POA or that Hermione chose to spend more time with Ron than Harry when they were fighting in GOF or that Hermione chose to go to Hogsmeade with Ron or at least went over to say hello when she saw him there in GOF.

Now if all that - and many other incidents - had been written with an R/Hr slant would Harry have just assumed that Hermione would for a start automatically go with himself and Hagrid? Afterall didn't Ron, their best friend, deserve support? If Harry had known, based on previous behaviour, that Hermione would have wanted more than anything to watch Ron compete he could have said "I'll see what he wants, you stay here and support Ron.' Remember this is the girl who made a point of watching Harry's quidditch match in POA when she wasn't talking to the boys and was then afronted when he asked if she had seen it. As if she'd miss an ordinary match of Harry's, yet here's a quidditch final? Doesn't Ron actually need as much support as he can get?.

Over the course of OOTP, Harry spends a lot of the time focusing on Hermione. Often with their debates they seem to effectively be the only two there. There is a major change to the pattern of the trio's debates. Before OOTP Harry tended to listen to Ron and Hermione thrashing out points and then choosing which one he preferred. In OOTP the debates are largely between Harry and Hermione with limited input from Ron.

We get a lot of descriptions from Harry about what she is doing, her reactions and a number of references to her face glowing or eyes shining or to her smiling - more so than in previous books. He hears her voice in his head and she appears in his dream and in his occulmency encounters with Snape. These are developments in their relationship even while Harry's relationship with Cho reaches a climax and conclusion. He thinks of Ron and Hermione to produce a patronus against the dementors, he feels a "warm glow" inside him when he meets up with them, he thinks of them again after his dream about Sirius. (Note: the warm glow flares when he sees his friends but Hermione hugs Harry for a couple of minutes before he sees Ron so the glow starts when she hugs him).
Harry has always measured Cho's effect on him by how his stomach reacts. But in OOTP Hermione's effect on him at times is far more powerful, full-bodied: "the warm glow that had flared inside him", "a wave of hot prickly anger swept through Harry's body" "such a powerful wave of relief swept through Harry that for a moment he felt light-headed."
In short, she becomes even more essential and important to Harry over the course of OOTP.

While Harry goes through a period after Sirius' death of not wanting to be with people, that mood is eased somewhat by scenes of solidarity in the hospital where JKR brings back the group of six and on the train where she highlights the DA members and at the station where she highlights the Order members: all sources of strength to Harry.
So while the prospect of Harry wanting to withdraw appears strong with the fresh wound, that's lessened by the ending.
Essentially, for all these reasons it seems to me very unlikely that he would want Ron and Hermione to spend time away from him on their own should they get together in the next book.

HERMIONE:
Hermione has a long history of prefering to be with Harry over Ron, showing more concern for Harry over Ron and having a closer relationship with Harry than Ron. If in the unlikely event she decided she'd rather be Ron's girlfriend than Harry's, it is still impossible to envision her making Harry's welfare anything but her top priority.
Remember Book Six marks the start of the Second War: the situation is worsening, Harry will need Hermione more than ever.

What would Hermione have learned from the OOTP year? Basically that her instincts about and knowledge of and concern for Harry were greater than anyone else's, including Dumbledore's. She, unlike Dumbledore, knew Harry would do something rash if he were kept out of the picture at the Dursleys. She has also learned that mistakes and wrong advice at this stage could be very costly. For example, Harry, Hermione, Ron, Ginny and Neville could all easily have been killed at various stages of the DOM fight.
What other conclusion can be drawn from the Christmas scene where she puts considerations of a family Christmas and a skiing holiday with her parents aside to go to the help of her friend. Everyone else in the house is very concerned for Harry, but their concern is shown in ineffectual sympathy. Only Hermione has the passion to confront Harry, get him out of that room and on the road to recovery. She won't take no for an answer.

Throughout the book her actions make Harry's load easier: I'm referring to Hermione's idea to set up the DA which, Harry says, was the major factor in helping him get through the year, and her idea to get his story published which changed people's opinions about him in the school and outside and eased a lot of negative pressure on him.
I think her desire to help and protect Harry will make her even more determined to stick by him and to shoot down other influences or views that Hermione believes are counter to Harry's welfare. Look at the savage way she attacked Ron when he suggested that Snape might be making Harry's mind control weaker: she didn't want Harry to be put off learning occulmency.
This priority of Hermione's to ensure Harry's safety and support him has the potential to wreck her relationships with Ron and Ginny.

If we take the R/Hr shippers' line that both Ron and Hermione have feelings for each other, imagine that they acknowledge those feelings. What happens then? They become boyfriend/girlfriend rather than friends. Now normally one's partner takes preference over a friend: that's why they are your partner, they're supposed to be more important to you.

So either:
1) the trio's relationship stays the same with Hermione still focusing and fussing over Harry and relegating Ron to second best (she does that now with their friendship).
2) Ron/Hermione spend time away from Harry
Now with 1) as much as Ron loves Harry as a friend how would he cope with Hermione showing more concern, interest, devotion to Harry than himself? The situation is manageable as friends because that's the pattern of behaviour they are all used to. But when the relationship gains importance, expectations change. Why is she his girlfriend if she wants to be with Harry? Why does she put Harry's needs before his? Why is Harry the one she turns to for support in a crisis? All this would eat away at Ron. JKR has already shown the effect of Harry and Hermione's closeness on their relationships with Cho and Viktor, who both became jealous and suspicious.

With 2) is it at all realistic that Hermione would want to be off walking around the lake with Ron or going to Hogsmeade with him if Harry has some problem that needs solving? Look at the effort she put into the Rita article, the DA idea and coins: schemes centred around Harry. Is it at all realistic that she would want to be off with Ron, leaving Harry to be influenced and helped by someone else? Someone who would know him as well as Hermione and wouldn't care for him as much.

RON:
Ron was a background figure in OOTP seeming to be fearful of getting into arguments with Harry and lacking in confidence.
But we have now seen two flashes of jealousy/suspicion over his best mates' relationship: in GOF when he asked Hermione about love potions and in OOTP with his reaction to Hermione's comment on Harry's kissing abilities.
Some H/Hr shippers believe that he caught on quicker than Harry and Hermione did that neither were happy after Cho kissed Harry and that he was trying to suggest Harry to Ginny on the train to get Harry out of the way. His old jealousy over Viktor was ressurrected again and he acted in a possessive way over news of Ginny's boyfriends.
R/Hr shippers seem to believe that the trio's relationship would carry on as normal if R/Hr happens. They don't take the change in expectations into account.

GINNY:
R/Hr and H/G shippers point to the fact that Hermione and Ginny are friends and say that everything would be rosy if R/Hr and H/G happened. But at the moment Ginny is not a threat to Hermione's closeness with Harry because Ginny's relationship with Harry is on a much lower level than Hermione's.
Ginny is not Harry's equal. Clearly in OOTP Ginny is ready to take orders. When she offers Harry the chocolate, Ginny is incapable of coming up with any ideas of her own to help Harry apart from offering to go to the twins to see if THEY can think of something. Were's the initiative? She is unquestioning and unthinking in the chocolate scene. She gives no thought to the possible consequences. All she wants to do is help Harry achieve what he wants to do, regardless of how mad that is. JKR even writes when they arrive at the MOM that Ron and Ginny walk in the phonebooth "obediently." Harry does not need that in a partner. He needs someone who is strong enough for him to be able to lean on at times and whose advice he can trust. Remember the right vs easy theme? Hermione is the one who is right.

Look at the scene where Harry and Hermione are debating what to do about Sirius: it's almost like the two parents discussing with the kids hanging around in the background. Ginny agrees to help without knowing what it's about. She, Ron and Luna take orders from Hermione as Hermione works out a plan to get into Umbridge's office.
Imagine the roles were reversed and it was Harry debating with Ginny what to do with Ron, Hermione and Luna listening on. It just doesn't work does it? Yet isn't that what should happen if Ginny was Harry's girlfriend and Hermione was his friend?
For a start there wouldn't be a debate, they'd be discussing how to fly to the MOM. Would Ginny be able to offer any advice or alternative views? And would Hermione be content to keep quiet in the background if she disagreed with what was happening?

Harry wasn't easily impressed by Ginny in OOTP: he noted that he would have caught the snitch quicker than her in one game, he didn't want Ginny, Neville or Luna to go to the DOM. Yet despite all his knowledge of Hermione's correct instincts, intelligence, jinxing skills and planning ability he would go to Ginny for advice and support rather than Hermione? And if not how would Ginny cope with being second best to Hermione? Well considering she gave Michael Corner the flick after he got upset at her team beating his at quidditch, I'd say she'd have a low tolerance threshold.

Ginny has a few barriers to overcome to become important to Harry.
She's younger, she's close friends with Hermione and friends with Harry and they are both also friends of the Weasley family. She's the kid sister of Harry's best male friend.
Just how difficult is it going to be to wean Harry away from his two best friends - one of whom happens to be her big brother - to spend time alone with her?
Is Hermione going to take any erosion of her closeness to Harry lying down? Well clearly what happened with Cho suggests she won't.
Let's think of the practical implications: Both Ginny (and Luna) are in a year below Harry, Hermione and Ron. So the pattern of the trio spending so much class time and after hours study time together must continue unless there is a volcanic upheaval in their friendship. Ginny and Luna's magical learning and experience is at a more junior level than the trio's therefore they can't help Harry with studies or advice to the same level as Hermione can. And Hermione, as each book tells us, is advanced for her age, the best student overall of her year.
It's curious that JKR made other candidates for Harry's heart apart from Hermione belonging to different years than the two of them. Cho is a year ahead and in a different house, Luna is a year behind and in a different house, Ginny is a year behind and in the same house.
That practical barrier means Harry will spend most of his time with Ron and Hermione unless there is a split in their friendship. Since we've seen that virtually nothing will keep Hermione from Harry, any future split between Ron and Harry would most likely see Hermione spending most of her time with Harry. That's what she chose to do in GOF.