Say, you fine-tune (in 6DOF) your head position when looking forward, then assign this position to, say, POV-neutral. Then fine-tune looking backward over right shoulder, put it on POV-right-down, then left shoulder - POV-left-down, and so on. And these finely tuned positions of your head will be perfect as long as you fly, say, a Camel. But if you decide to drive a FokkerDR1, you'll find out these head positions are absolutely wrong due to different cockpit layout. You re-adjust for Fokker, but when you return to Camel, everything is wrong again. Get what I mean?..