I wonder as I wander... POV musings

Edit/Quick Note: I'm going to be moving and house-petsitting at a place without Internet, so if I don't reply for a day or two, it's not because I'm not interested, it's because I've gone wireless. Woe. But I'll try to check in when I can. Thank you all who have contributed your thoughts; evidently I'm not the only one who's pondered this tricky topic!

I was recently beta'ing something for someone and I pointed out how her POV had suddenly switched from character x to y. When she wrote back to me, she said that she didn't understand why I was upset at the inclusion of character y's POV. This really got me thinking (surprise! lol). I'm a music history/theory major, not English. I couldn't punctuate dialog when I first began writing because I'd never written fiction. But my first beta was good at pointing out to me when I would suddenly switch POVs. Now when I did it, it was a mistake; I didn't know I'd accidentally written about something that character x couldn't know because s/he wasn't in character y's head. I was in both character's heads; hence the confusion. I've read some stories by people who I know know what they're doing, and they switch back and forth anyway, without visual paragraph indicators that say to me, as the reader, "Hey- I'm switching POV and I'm doing it on purpose." It made me cranky. I actually kept reading one story like this, but I grumbled out loud at the author as I did it, telling her she knew better.

I guess my question is this: is it 'old school' to feel that once you start your story in one person's POV that you should stick with it, even if it's difficult? Especially when you're really dying for your readership to know what character y is thinking, you just suck it up and keep going for the good of the story (and work harder to show it, which I think is the stronger storycraft)? When I look at people's stories in which the POV flits back and forth, there are usually other indicators that it's probably a relatively new writer. To be honest, when I see it, unless it's in a story by someone who I know has been writing for some time (at which point I either want to throttle them or think perhaps it was an honest mistake) I immediately think: "New Writer. Doesn't know any better." If it's in somebody's story who's been writing for a while, I think: "Lazy." Or, "Nobody has told the poor dear that you shouldn't change POV. If you *insist* on knowing what each character is thinking, use third person omniscient or whatever it's called." [please see part about not being an English major]

But is this kind of thinking passé? Is it a trend, using willy-nilly POV, kind of like writing in present tense instead of simple past? Of the things that really really jar me out of a story, besides homophonic errors, is POV switching. I feel you do your reader a disservice by suddenly jolting her/him out of one character and into another, but I realize that other people don't mind that at all! There are also people who are used to composing their stories as though taken from an RPG, in which wandering POV is required. You won't be surprised to hear that I don't read much of that.

For you grammatically-inclined people, what are the historical rules? Are you being lazy and a bad writer if you don't stick to one POV? Or am I simply a stick in the mud? Luckily there's enough fic for all of us, no matter our preferences.