My uncle is a class III dealer and therefore I have shot my fair share of full autos. Other than being fun to shoot, I think they are a complete waste of ammo. I was once at a machine gun shoot in Arizona when I was like 13 and it broke my heart to see all the ammo wasted there. As far as NFA weapons go, I would rather own SBSs or SBRs or suppressed handguns. Those are more practical IMO.
In other words, if you are advanceing on an enemy position while they are shooting at you 1 or 2 of your guys can start shooting full auto on the enemies position and basically make them stop thier shooting because they need to take cover as to not get shot and your men can more easily advance. (Someone else may be able to word this better. I do not claim to be the best in the world at useing words. )