With the interior of any tank already cramped and filled to the max, there is not, nor there ever was room for an extra "roof gunner". So while the pintle-mounted MG is manned, someone - commander, gunner, loader, radioman - is not doing his job. In any case, on those rare tanks where it was installed, the roof MG was used mostly for AA purposes. Of course, it could be used against infantry, but would you really get halfway out of the tank to expose yourself to small arms fire, if not absolutely necessary? (Staring into the guns of a Typhoon or FW 190 armed with a light machine gun could be said to be equally self-destructive...) Some modern tanks have remote control for the roof machine guns to make this practical but such luxuries were certainly not available in WW2. So it made sense to more or less permanently "loan" the gun to supporting infantry.