This is indeed a thorny issue. TIR is quite an advantage, but there are lots of folks who do not use it that are supremely successful in aerial combat. One of my squad mates, BlitzPig_Raven, uses his mouse with his left hand to look around and he is a
deadly opponent.
Personally I don't see 6dof as a deal breaker, and if you are honest about it, it does let you look around in ways that a real human could never do, unless you had a meter long neck made of rubber.
