Here's a good page about the 1994 ban:
http://www.guncite.com/gun_control_gcassaul.html
These aren't machine guns. They're typically semi-automatic (one pull of the trigger discharges one bullet) guns with "scary" cosmetic features like folding stocks, mounts for a grenade launcher and pistol grips. To be fair, it's reasonable not to have folks running around with grenade launchers, but as far as I know it's already illegal to possess grenades without some kind of explosives license so what good does banning such a mount do?
"Assault weapons," as defined under the 1994 law, are involved in an extremely small number of crimes (about 0.2% of violent crimes and 1% of gun crimes). They're not exactly the preferred choice of drug dealers and gang members because they're not concealable.
If you truly want a machine gun, you can still have one. You'll have to pony up some cash due to their scarcity. The manufacture and import of machine guns was banned in 1986, but they're still legal to own and transfer as long as you pay a federal tax on them (I think it's around $200) and have the proper paperwork.
So why do politicians want to ban assault weapons? Because they want to appear to do "something" about gun crime and doing "something" about a gun that sounds scary is even better. Who do you think first started calling them "assault weapons?" Not gun enthusiasts.
Furthermore, banning a single class of weapons like "assault weapons" is a good first step to banning other kinds of weapons. Hell, even the Washington Post recognized that back when the first ban went through:
"No one should have any illusions about what was accomplished (by the ban). Assault weapons play a part in only a small percentage of crime. The provision is mainly symbolic; its virtue will be if it turns out to be, as hoped, a stepping stone to broader gun control."