One other aspect of why we are against the often seen "shooting while yelling PC commands" is that it prevents you from shooting well. Gunfighting is a martial art and rather than minimize the study, we go the other way. Like athletic performance, we want to do everything possible to insure the surgical shooting we want to accomplish. Any visual study of top athletes reveals that they keep mouths closed when exerting. Either explosions of power, speed, or strength are made easier with a closed mouth. Moreover, watching Olympic archers, or athletes that require exactness, position their jaws and mouths the same way.
Partly it has to do with clearing the airway, and partly has to do with optimizing muscular-skeletal positioning.
Here is a great article on Charles Poliquin's blog - Strength Sensei