Well, I do turn my head a bit - maybe 10-20 degrees to sides - when I play some FPS. Just imagine yourself, e.g., strafing out of the corner. Supposedly your EYES are still locked on display before you, yet you can rotate your head a bit. And this should mean a lot in such a detailed environment: make all these efforts to produce a precise acoustic environment and then to fail by ignoring head direction - so that sound would be inconsistent to picture on a fixed monitor before you? BTW, when I was younger, I played Counter-Strike, and on some maps you could camp nicely behind a wooden wall, hear someone coming on the other side and precisely shoot the guy with a few bullets. They all were shouting "cheater" for the first time, thinking you can see through walls... Nope. But when a man tries to establish precise source of some sound, one often turns his head to sides a few times. Something related to binaural hearing and psychoacoustic model, perhaps.