Actually, it's not that bad of a target - if you break the pelvic girdle, the attacker isn't going to be able to stand, much less walk.
Of course, in any discussion of where to aim, one has to remember that one only fires in order to stop the threat. Civilians aren't authorized to "shoot to kill" although that might happen. Lethal force can only be used in self defense, at least in Michigan, until the threat is stopped.