In my point of view, having the green bubble indicator always on is not a good idea. Imagine a situation that you are in combat in serried buildings, it would be much of an disappointment to know which builidng is full of friendlies and enemies before actually going in. Having the indicator only visible when aiming may be better. However, I think teamkilling is not much an issue after players get used to the game. Afterall, a player should be able to distinguish the uniforms, and if not, make his/her decision to fire or not.