Some servers might have the reflect mod
Basically, any damage on friendlies gets reverted on them.
Archers take the damage and get interrupted, not you.
Tkers kill themselves when they deal what would have been a death blow on you
The negative is that, lets be honest, in the thick of the fight, odds are at least once you might come close to, or even hit your teammate.
Accidents do happen.
Also, it takes away some of the realism of the game, if that stuff bothers you.