First of all, let me say that I am NOT good with a machine gun and have never been trained in the use of handheld full auto weapons. That being said, I own a submachine gun.
I agree with what you say. I always felt this way, but the point was really driven home to me during a local machine gun match I was participating in. The match director tries to come up with something a little different to keep it interesting. One match you had to start off with a rifle, knock down something like five or seven steel reactive targets, then put the rifle down and pick up your subgun and continue on with two other target arrays. I decided to use one of my AR15s with an Aimpoint. The rifle targets were around 50 yards away and I knocked them all down with one shot each in a few seconds. When I went to sit the rifle down, I realized how much i wanted to keep it. I knew I could complete the other two courses of fire much faster with a semi-auto AR15 and an optic than I could with my sub-gun. I also could complete the COF with a fraction of the ammo, and would be firing a much more effective cartridge. If the situation had been real and I had the choice of the semi-auto AR15 or ANY subgun, It wouldn't even thing about taking the subgun.
I recently got caught up in the idea of buying another machine gun. Specifically I was going to buy an M16. In discussing it with a couple friends who also own full auto stuff, I realized that there were a couple reasons I wanted to buy one. First of all, it is the forbidden fruit thing. They are not real common and take a little effort to own so I guess it makes it attractive to me for some reason. Another reason is the Gee Whiz factor. I have no real interest in firing the thing full auto. If I buy one, I will have to give it a try but that will probably be the one and only time I ever fire it on full auto. It just doesn't do anything for me.
You know the rest. In the books you have read
How the British Regulars fired and fled,
How the farmers gave them ball for ball,
From behind each fence and farmyard wall,
Chasing the redcoats down the lane,
Then crossing the fields to emerge again
Under the trees at the turn of the road,
And only pausing to fire and load.