I've read about this objection multiple times:
Thieves encourage conflict within the party by stealing from other PCs.This is the objection, in contrast to my last post, that I understand the least. As a player, if a thief steals from my PC, my PC will retaliate, and I assume that the rest of the party will back me up. As a referee, I don't see any reason why I should stop that. If the thief's player pulls some "that's what my PC would do" spiel, well, that's also exactly what the other PCs would do too, now isn't it? The thief is outnumbered and out-matched in combat, if not against the fighter, then definitely against the wizard. The player, and the PC, are going to learn a quick, hard lesson about stealing from the other PCs. If the party turns the thief over to one or the other elements I control as a referee, I would probably make sure the book got thrown at the thief PC. If the thief's player wants to bring that kind of thing into my game, they should know that there are consequences.
Now, call me naive, but, so far as inter-party conflict goes, this seems pretty cut-and-dry. I'm personally more concerned with the way a party splits up loot (do magic items count as a part of the pot that we split or do we just give them to a PC that can use them since they help all of us? do we give loot to a PC that wasn't there when the loot was looted?), as that's what I've actually seen cause inter-party conflict. (The best answer for the loot, by the way, seems to be having the party decide the answers before there is actually any loot to fight over.) Of course, I've never actually seen a party member steal from another party member, so I might somehow be wildly underestimating this.
Have you ever seen this in a game you played in or refereed? How did it get resolved? Was it that big of a deal?