Well, for one, the ability for the populace to be able to overthrow the government if the need should ever arise. Granted, an Uzi will do little against a missile, but if there were enough people upset that an actual overthrow were possible, I'd rather have a full auto than a sharp stick. Besides, what's the point in banning them? As long as the police and military can get them, so could any crook. Not to mention, automatic rifles are very very rarely used in an actual crime. (Except in the movies) Pistols are MUCH easier to conceal and they can kill you just as dead.
Don't get me wrong, I have very little use for an automatic, but if I ever do, I'd like to think I could get one. Besides, automatics are MUCH more difficult to handle unless you've been trained. I'd just as soon go up against a random thug with a machine gun as one with a .45. Believe me, I've shot a few auto's, and it is NOTHING like the video games. I know what to expect and I STILL can't keep the thing level. Even on three round bursts. The first shot may hit the target, but the rest are in the big dirt pile behind it.