You won't have time to hesitate and think 'oh, I better aim for the leg' while he's trying to kill you. If he has a gun or knife, you'll be dead by then.
One thing I've noticed is that anybody who claims that one should shoot to wound, say, target the leg or shoulder, or even better, shoot any weapon they have out of their hand, have never shot a handgun before.
Shooting a handgun halfway decently is not that easy. It's an acquired skill. The vast majority of people I've taken shooting for the first time can't even hit a human size target 15 feet away. Just the way they pull the trigger and brace for the recoil throws everything off. And this is under a stress-free controlled environment.