I don't like enemies running at me with an arrow sticking out of their knee and when I smack someones unarmoured head with a battle axe I want more damage than if I smacked their left hand.
I think it would also add slightly to the way you play the game such as looking for exploiting unarmoured areas on persons or monsters having 'weak spots' in which they become damaged more such as the undercarriage of the spider or the eyes of the troll. Well you get the general gist. I don't want it solely for the sake of 'realism' but because I think it would add a new dynamic to the game. Imagine crippling a leg of an enemy in order to flee if you are in a tight situation and of course seeing your prey desperately limp away is priceless.