Too many people would grief just because they can...and don't care about the consequences. How's that fun for the guy that got shot down through no fault of his own?
Communication on range...there's a concept. If you see the guy diving in, stop shooting. If he would have given warning that he's making a run, you could have stopped shooting. Yeah, it sucks when you killshoot yourself because someone is trying to clear your 12 O'Clock, but to fix it you just open the door for the type of people I mentioned above.
Just like the majority of "problems with the game", the root of the problem is human nature and the way people behave in the anonymous world of online gaming.