I'm getting to ready to write a fairly emotional scene this afternoon, dealing with grief and guilt. Last night, I fell to pondering whose point of view I should use, the hero or the heroine's. This became a question because I've just written a scene from the heroine's point of view. Then I fell to pondering the general guidelines I use for POV decisions. (I say general guidelines because I break my own "rules" from time to time -- I'm telling a story, not baking a cake.)
One of the things that really appeals to me about romance is that I have two strong characters with their own points of view to work with and that they should be fairly equally represented. That's one reason I shy away from first person, both in my writing and reading. Given that our readers are mostly women, I'd say if I had to tilt the balance more in one direction, it probably should be the heroine. However, I love strong, interesting heroes, so if the balance is more in the guy's favor, no complaints from me.
But the main consideration I use to decide whose point of view I'm going to use is who's going to have the greater emotional reaction to the action of the scene? Who's more "invested" in what's going on?
That's why, although I've just done a scene from the heroine's point of view, I'll probably stay there for this one. What's happening is going to hit her much harder than it would the hero.
Do I ever give secondary characters a point of view? Oh, heck, yes, especially the villains. I like to know why they do what they do, even if it's just because they have an enormous sense of entitlement, and I think being "in their head" is a more interesting and better method than having them suddenly start explaining themselves. And this way, I can show what they're up to when the hero and heroine aren't around, which enables me to create tension and suspense.
What about switching point of view in a scene? Do I do it? Sometimes. If I think it's more effective in terms of conflict and drama and plot development, I'll do it. But I do think about it.
I believe one reason many people will tell you to stay in one point of view per scene, or even chapter, is that so many people do the change-over poorly, or too often, so that the reader is pulled completely out of the scene. Ideally, the transition should be so smooth and so good in terms of telling the story, the reader doesn't even notice.