What is it exactly that you wish to do with an automatic weapon? Please explain.
There are any number of traditionally legal uses for a gun; an automatic weapon is suitable for use in any number of them.
You can't go hunting with it....
I hunt with an AR-15 - why can't I hunt with an M16A2?
doesn't make you any safer in defending yourself or your property
I have a semi-auto M1928 Thompson for home defense -- how is the full-auto version any less effective to that end?
The only reason for a person in our current day society would WANT an automatic weapon, is to commit such atrocities like what happened in the movie theater in Denver.

That's the ONLY reason, eh?
Explain how/why none of the legal machine guns in the US have been used for this purpose?