Wore one for ten years straight with my backup gun. Still use one regularly.
As you start your draw, you don’t stand there like a mannequin, you either step forward with your off leg or drop your primary leg back into a fighting (Weaver) stance. Your off hand is brought up to fight, to defend your head, etc, between the two you have just reduced the movement of the gun by 80-90 degrees. This also moves the arm out of the line of the draw as well as reducing the distance you have to reach for the gun.
As the gun is drawn, the muzzle and the arm is dropped and the wrist twisted to bring the muzzle on target. It’s surprisingly quick and easy with a little practice.
Depending on circumstance, such as this AM where it was -9F and belt holsters are buried under layers of insulated clothing, the shoulder rig may be the only practical means of carrying a fighting gun.
At muzzle contact distances, an aggressor can just as easily get a hold of your belt holstered gun, especially from behind. And one thought, the first place bad guys (and good guys) look for guns? Behind the right hip. The Cops and most CCW carriers carry their guns there.