I have a few simple concepts:
1) Any law should affect criminals more than non-criminals.
2) The maxim of "your right to throw a punch ends at my face".
3) The oft-quoted "rights have restrictions, you can't yell fire in a theater" is inaccurate. The fact is you're responsible for the results of your actions. If you yell fire and people run and hurt themselves, you're responsible. If you yell fire in a comical moment and everyone laughs, you've broken no law.
So, no one should have to justify why they NEED something, whether it be a machinegun or a corvette or a boat or a super big gulp. Like alcohol, you don't need to justify why you want the $50 bottle of wine instead of the 4 pack of natural light, and so long as you don't bother anyone I don't care. Cigarettes and explosives are harmful in the normal course of their use, so restrictions make sense.
So yeah, it makes sense to have laws saying you can't shoot across the street at a stop sign, no matter how fun that might be.
It does not make sense to ban something that was regulated from 1934 to 1986 and had no measurable criminal use during that time. You don't NEED a machinegun (though if you're saying the 2nd is about keeping the Militia well armed/ trained, then it seems like the M4 should be the main gun allowed...), but you also don't NEED to ban them, so they should be allowed. Go ahead and make them more expensive perhaps, a $1000 tax for each one made would keep out the plebs, but there's no point to restricting them.
Of course, if you reversed it today, the press would result in a huge surge, so they probably would be used in crime because of all the news, so bleh.