In general i like to have friendly fire, because it makes the game experience more realistic and you have to be more skilled not to shoot your friends while a hard fight.
And i agree with others, it can be surely annoying.
But we have to think about, that it gives us much more possibilities to interact.
I can imagine to have some Debuff for harming a friend, even a Crime Flag, if you kill a friend that will make your life more complicated.
I always missed the possibility to shoot to your own faction, if there where someone who is really annoying.
With friendly fire the funny kiddies who want to prank you without fear of consequences would think twice before they start to annoy you. 