Generally speaking, you don't have to play nice with your teammates. However, it will benefit you greatly if you are a nice guy/gal, play along, and generally don't be a jerk. Why you ask? Simply put, I'd be more than willing to give the nice guy of the group something I picked up vindictus gold from alpha loot rather than give it to the jerk.
So as you can see, if you are looking for that special buy vindictus gold, your chances of getting it increases 5X? It always pays to be the good guy.
Next reason why you should play nice is, for the simple fact that there are ALOT of shit lists out there, and being on them isn't going to get you that last spot in the uber group. There are two things that people remember the most in this game, and that's:
1. Who the nice people to team with are.
2. Who the real asses to team with are.
If you fit in #2, then you are S.O.L. and should have no excuse for crying and whining about why your so much more l33t than the nanomage enforcer who just took the last spot. You will have nobody to blame but yourself.