The reason you'd go right against a window when you stand-aim an mg down the sights is the same reason a rifleman stands against the wall - Cover, and a greater swathe of area covered.
Also, no need to change the brens. They suffer much lower spread than mg34/ 42 hipfired and they are also deployed quicker.
The system is designed to give you NO practical advantage using it in wide open area... you wont hit a dinosaur save by blind luck. The real use is to behind objects like walls, desks etc, where the best use is firing down the sights and 'lying' in wait of enemy.
I was also concerned at first, but preliminary tests have it as quite functional a system.