Let's look at it from a metagame point of view.
The classic D&D PC group is founded upon mutual cooperation for the survival of the group. Yes, in the near 25 years I've gamed, I've seen infighting and (literal, in the game world) backstabbing amongst characters based on the lack of such cooperation. Much of it that I've seen resulted from behavior that has been justified as stemming from roleplaying. The problem is - and bear in mind that this all anecdotal, so I'm not saying it's universal; but I've seen it happen in various groups from different places over the course of two and half decades - I've never seen anything but hard feelings result from it.
Anyway, once players realize that they can't rely on, or trust, another player to at least attempt to pick up the slack with his character, the fun of the game becomes diminished for them. That's my experience.
Roleplaying is fun, I agree. However, I think players should try to adjust their roleplaying goals to fit the group they choose to game with. If they don't wish to do so, it may be worthwhile to seek out another group whose goals more closely match one's own.
I was recently playing in a Call of Cthulhu d20 campaign. One player, who was, and is, liked as a person outside the game, was hated as a player while playing. His PC's refusal to work with the rest of the group consistently caused problems, to the point that two PCs were killed as a direct result (including mine), effectively ending the campaign for those players. The rest of the players (including the Keeper) seem to have finally gotten fed up, because the last time I checked (I don't go every week, being dead and all

), another PC finally killed the PC in question. Hard feelings, of course, resulted. The player
did remain consistent in his roleplaying, as did the rest of the players. The trouble was, it resulted in people not getting the kind of enjoyment from the game that they wished to get. Which reinforces, in y opinion, the need for players to adjust to the group they're in, or find a group in which they don't need to adjuest as much. It's a game, after all, and everyone is there to have fun.