If someone pulls up data showing me assault weapons have caused more unintended or malicious damage than other legal guns, I would be swayed to that side. If you are going to go around banning things that cause harm when they are used in an illegal or improper fashion, there is a whole list of things you are going to have to end up banning -- like cars, knives, golf clubs, bats, hammers, pipes, etc.
I think the idea is to ban things that cause MORE harm than good. There is the idea that standard guns are enough to defend yourself against criminal and personal attack. And that the harm caused when automatic weapons are used wrongly outweighs the benefit they have as a form of defense. You don't need a lot of rapid-fire bullets to kill an attacker. But they sure help you if you want to kill a lot of people in a spree.