Alicia, the topic of switching POVs in the same scene is a touchy one and I would love to hear your thoughts on the difference between shifting those POVs for "a good reason" in a scene vs. head hopping and and those "good reasons" are. I get into arguments about this all the time with my fellow writing friends. I tend to be a one POV per scene kind of girl, but I've been known to shift halfway through and slide into another (say from hero to heroine) for the rest of the scene. Some argue such "hopping" is their style. So how do you try to distinguish between a shift for a reason/doing it well vs. rampant shifting?
I also tend to be a single-POV type of gal, but some of my favorite writers are multiple POV writers, so I have to admit-- that can be done well. (I also have to say this-- it's discussed in my Writer's Digest book. :)
First, let's distinguish between omniscient and multiple POV. Omniscient can shift from mind to mind, but with omniscient, there is a controlling "above" narration, a mentality (not necessarily a person or the author) which knows more than the characters individually or collectively know. Most non-first-person British stories in the 19th century used omniscient point of view ("IT is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife. However little known the feelings or views of such a man nay be on his first entering a neighbourhood, this truth is so well fixed in the minds of the surrounding families, that he is considered as the rightful property of some one or other of heir daughters."), perhaps because of the Victorian reverence for authority-- the omniscient point of view is all about authority.
But the multiple point of view doesn't have that controlling overall authority. The narration is always in the head of one character or another. All the information and knowledge and perceptions of the narration come from the characters-- there's nothing (in the multiple POV passages-- there is nothing wrong with omniscient passages, especially at the beginning and end of scenes... remember, the only rule is, it works if it works... just know if it works) that is "above" or "inserted" from a more knowledgeable non-character. This makes it like single-POV, that is, the narration is confined to the character immediately in viewpoint (just in single-POV, it's just in one character).
Now your question is what are good reasons for using multiple POV.
1) Let's start with the biggie: It's right for you. It's the way you think. It's your natural POV.
I can't stress that enough. You have to write from your own mind, and if you naturally write in multiple POV, why fight it? Better to just do it really well so readers and editors will get a great experience. Now that most people grow up not just reading but watching TV and film (which are usually in multiple POV, if they can be said to have POV), it's not at all "wrong" that the more visual writers, the ones more cinematic in their mentality, are likely to write in multiple. Go with it-- just do it well. And ignore all those single-POV puritans who act like you're sinning. You're having more fun.
2) So that's the best reason, that it's your natural approach. But there are other reasons, and even naturally-single writers can occasionally use multiple to their advantage. Second good reason-- to juxtapose the understanding or perceptions of different characters. This is one of those LESS IS MORE situations, by the way. If you're constantly switching from one character to another to show, for example, that Joe loves the curry and Mary hates it, it's just going to feel incoherent and bipolar. But if you want to juxtapose something interesting and important, to show that two characters have different understandings of the current reality, multiple is a great way to go. For example, I am working on a scene where the protagonist and his best friend see a couple walking hand in hand. The protagonist has a secret crush on the lady in the couple, but the best friend assumes his interest has to do with the illegal activities of the young man. Switching from Pro (who is longing for the lady) to the the clueless best friend not only shows the difference in their understanding, but also something about their relationship-- Best Friend might not know Pro as well as they think, and Pro might not trust Best Friend all that much. I thought it might be even more interesting to have the best friend cavalierly point out that he's surprised that a cool dude like the young man is wasting his time with the lady, because she isn't very pretty. This of course is going to get some reaction from Pro, but I've given him reason not to be honest about his feelings. So I might consider switching to his POV to show his internal conflict-- he wants to slug his best friend for dissing the lady. Then again, I might stay in the best friend's POV and show him misunderstanding yet again, this time interpreting the Pro's sudden tension as evidence of fear of the illegal guy. Anyway, juxtaposition will show the difference in knowledge and in perspective in a vivid way.
3)This is related -- multiple POV allows more perspectives in crowd scenes, such as, oh, to be timely, a group of disparate people watching the inauguration. The old civil rights worker who never thought she'd live to see this day will have a different reaction than the McCain voter who thinks the country made the wrong choice and the snarky ironist who is contemptuous of all the enthusiasm around him. Now of course it has to be important and worthwhile to "sample" all these different perspectives. But if you have decided you want this, it's much more efficient to do it in multiple than to have a scene-shift with each shift in perspective, that is, show the whole event from the civil rights worker's perspective, then do another version of a scene from the next perspective, etc. That method-- single-POV multi-scene-- is bound to get repetitive. This sort of "crowd control," btw, can be used effectively in some comic scenes, to show how different characters apprehend what's going on.
4) Multiple POV can be useful in controlling larger settings or multiple settings. Actually, I tend to think that when you shift from one setting to another, you're probably starting a new scene, but going with multiple POV will allow for more expeditious narration of events happening at the same time in different places. This is almost impossible to do effectively in single POV (we generally have to bypass the multiple-event challenge with some single-POV trick), but it can produce tense scenes and a lot of action. Perspective can shift whenever action starts to lag.
5) POV-shifts can delay gratification to create suspense. This is not about shifting to a new POV in order to give a trivial, momentarily spurt of gratification, but actually shifting AWAY from one character so you can hide something (like a reaction or a thought) that later will be revealed in a different way. That is, POV-shifting can be a form of information control, keeping something from the reader without cheating (as it might be sort of cheating to stay in the character's POV and not report some thought or reaction).
Those are just a few good reasons. But really, the truth is, you don't generally choose your POV-- it's a reflection of your own approach to writing and the kind of stories you choose to write. That is, it's all connected. If you're naturally interested in stories with a lot of action and multiple settings, you're probably just not likely to be a single-POV writer. I'd also suggest that the worldview that inspires writers to tend naturally to POV will differ. Single-POV writers are more likely to think that what's important is how reality is perceived and experienced inside a character, while (I think) multiple-POV writers are more suspicious, perhaps, that reality is an absolute-- rather believing that only by getting a collage of different versions can the reader begin to understand what's really going on.
Now... headhopping can't be dignified with an assumption of its worldview. It offers no worldview beyond chaos. Headhopping isn't about control, suspense, reader experience, character exploration. It's all about instant gratification for the WRITER, and just on those grounds, it should be rejected by good writers. (Good writers should be far, far more concerned with the READER.)
However, multiple POV by definition isn't headhopping. It's the controlled and purposeful presentation of more than one viewpoint on an event. Each shift should give the reader some benefit from the transfer into a new mind. And while I'm not saying you should always be conscious and analytical about everything you do as a writer, I do think that after you write a passage, you should probably be able to elucidate why you made each shift-- in retrospect, what the purpose of this shift is. (And if you can't make a case for it, I'd suggest reconsidering the shift.) That is, go with your inspiration, ride the wave, write the wave-- but when you revise, get analytical. Maximize the power of shifting by minimizing the occasions-- Less is more. Shift because you need to in order to create an effect, not just because it feels good.
So multiple POV gives me more options for scene presentation, but of course also more responsibility to know what effect I want to have on the reader and how best to attain that.