That’s for the “main” assaults like here on the objective. I won’t build the whole MG nest for some random skirmish with 2 squads. Nor will I build it in highly dangerous area like in view of enemy tank spawn.
Sometimes you need pure firepower, sometimes it’s better to have less firepower but more mobility.
Also personally I’d buff the hell out of MGs if they were mounted only*. Like suppression, reload speed buff etc. ideally together with assistant gunner. One can dream.