Dog bites can be a very serious injury. I've been bitten by a "friendly" dog that "doesn't bite." Hmm, Tell that to the puncture wounds I had in my leg. Problem is, dog bite wounds shouldn't be stitched, so sometimes they heal very slowly and stand the risk of infection. Plus, dogs can have rabies. Never any fun to deal with that!
Also, our dog (who was on a leash) was attacked and seriously injured by a neighbors dog that was not on a leash. On several occassions, my family and I have been charged by dogs! How the heck do we know if they'll bite or not.
In my opinion, based on my experiences, dogs approaching any human aggressively should be stopped by any means necessary. Pit Bulls should especially NEVER be trusted, no matter how sweet their owners think they are.