MP-38/40 Had unique rolls in the various German militaries.
The basic infantry unit in the German army was a maniple centered around a medium MG, with riflemen in support of the MG. The squad (maniple) leader was armed with an SMG This weapon was a badge of office, a fire-direction, device, and a tool in offense and defensive maneuvering.
For other armed forces, the SMG was a way to to increase (potentially) the number of rounds on target when used by infrequent, or less-well trained shooters. In such a case it was a tool of intimidation.
Now, if the Germans had access to the M2--select fire--model of the Carbine, then, that might have been a different sort of thing.
Except, they probably would have adopted some German version of 9mm longo.
It's good t remember one of the things that delayed issue of the STG/MP-44 was the reluctance of the German General Staff to introduce yet another ammunition into the supply train (they has something over 10,000 million 7.92 already on hand).
Personally, I find the Carbine points and carries far better than the MP-40. That latter is, though, supremely compact. It sling carries, on patrol, far better than the Carbine (although both are equally stabbity slung in marching order). Magazine carriage for the Carbine is far superior, both in access, and variety, of carriage. (Fumbling about with leather straps and buckles while prone--and wet and cold--will demonstrate the superiority of lift-the-dot and snap fasteners.)
But, I could be biased in this.