To start with, a fully automatic firearm is difficult to obtain legally and is expensive. There are relatively few of them when compared to everything else. In terms of criminal acts, they are seldom used despite what you see on TV.
The so called assault weapon ban does not address these types of guns at all, it addresses semi- automatic rifles, which require you to pull the trigger for each shot fired. That is what has people upset. If they can ban semi- auto rifles, then semi- auto shotguns and pistols won't be far behind. I doubt that there are many gun owners who are pushing to have the rules for full automatic firearms relaxed, but there are a lot of us who do not want to see additional gun control laws.
There are over 20,000 gun control laws on the books in the US. Adding more will not reduce crime. No matter what you ban, if the criminals want it they will get it, just like drugs and even other humans that smuggled into the country.
As for what a civilian would want a fully automatic machine gun for, well the 2nd Amendment does say that the reason we are armed is for the security of a free country. The word militia is prominent and that means fighting in battle. Hopefully that will never happen, either against foriegn invaders or tyrannical government, but if it ever did, as it has in many other countries then a fully auto weapon is exactly what the 2nd Amendment refers to.
You state;
"The 2nd amendment is important, I know. It was also written at a time when people couldn't even imagine a gun that could fire more than one bullet without having to reload in between shots."
True, but then they couldn't imagien electronic media like we are using either. Mechinism where biased reporting can form US and even world opinion. But would like us to use that excuse to have all news outlet go back to hand run printing presses and criers in the street?