This concept that limited POV is vital (or we slip into the hell of "head-hopping") is very big very suddenly. I'd call it a lit fad, like "multiPOV's are the new adverbs".
Usually those things are blathered about among people with little actual experience, and snowball up on re-telling.
I'm not John, but the first person who ever mentioned to me that it was desirable to stick to one POV per scene was my first editor, Terri Windling. She's won numerous awards, so I wouldn't call her someone with little actual experience.
That said, I agree that it's one of those things, like adverbs (I am a firm friend to the adverb), that people can get unnecessarily hung up on. Older authors used to handle frequent POV shifts so gracefully that nobody noticed. But it's hard to do right (and when it's done badly, it does give an impression of hopping from head to head), and as such it's something that inexperienced writers should probably avoid. I think it's one of those things that most people only notice when it is done badly -- otherwise it goes right past them.