Well, a semi-automatic AR-15 (the first firearm I plan on owning, be it in the National Guard or as a civilian) is just as good as a fully-automatic one.
Besides, as "Enemies Foreign and Domestic" showed us, a single shot in the right person's head sends a good message to the anti-gun fascists.
You make good points, except that the experts in our armed forces seem to think that full auto has benefits justifying arming most soldiers with such rifles.
If you had to cross a field with hostile shooters nearby, would you rather your pals have machine guns or semi-autos to provide supressing fire?