It doesn't have to be about challenging someone's decision so much as discussing the decision. Not "your character wouldn't do that" so much as "why did you decide your character would do that?" or maybe even "I was surprised that you had your character do X".
I think these kinds of conversations are interesting, and they can be helpful. They can also, of course, go wrong if people don't handle the situation properly. If it's an open session to question every other player's choices, then yeah, that's probably not productive.
Agreed. It's 100% about whether the motivation/attitude going in. If the point is to school the player on proper roleplaying, then no. Just no. But if it's more about "hey, we obviously are looking at things differently, and maybe a conversation will help us both learn something " then that makes a lot of sense.
Actually, that would also apply to threads like this.