Phil_Ozzy_Fer says:
My argument for alignment comes from experience at the gaming table. People have too readily enjoyed or expected to play morally ambiguous anti-heroes, and scoffed at the idea of their PCs having principles to uphold. This is the kind of thing that fuels PCs being murder-hobos. Theft, torture, and assassination are things players have tried to justify as being completely permissible for their characters, and the whole table agreed these behaviours were perfectly reasonable. With no alignment, this becomes the norm.
Interestingly, this has seldom been a problem at my table. Very early in my gaming career (when I was still using alignment, naturally), it occasionally reared its ugly head (particularly when one party member was playing an assassin and took it upon himself to try to trim the party), but I've found that a good core group of players will police themselves. I'm reminded of a game that I ran at a local game store several dozen years ago.
A new player asked to join the party. Now, I interview prospective players for any red flags, but this guy was slick. He calmly answered my questions and set my mind at ease. The party allowed him to join them, and the very first night, while he was on watch, he robbed them blind. He gathered up all their magical equipment (using
detect magic) and loaded it into a magical box that would instantly transport the gear to a second magical box many leagues away when the lid was closed. His accomplice, waiting at the receiving point, would then unload the receiving box for the next transport. All was going well for them until he tried to steal the barbarian's bunny slippers, thinking they were magical (they weren't; they had a permanent
prestidigitation on them to stay clean and in good condition). The barbarian awoke, and the party quickly sorted out what was going on.
The group agreed not to kill them if they returned their magical items. They had the thief lead them to his accomplice. Then, the barbarian stuffed both of them into one of the boxes, and the party mage sealed the receiving box shut with magic. They then destroyed the sending box and dropped the other down the deepest well they could find. Problem solved, and I didn't have to break DM neutrality.
The two players who perpetrated the theft were understandably upset, but the entire shop gave the party a standing ovation. These two had done this to other groups as well. They were asked to leave the shop and not return.
Anyway, the takeaway is that the party, which consisted of alignments ranging from lawful good to chaotic neutral, came together to solve the problem themselves. Loyalty to one another superseded alignment.