I suspect that flying faoff with relative mouse is easier (and actually it is).
But current faoff model must be considered as a game mechanics, and it is the game that everybody loves here.
I agree that some efforts required to learn to fly with this and still it is possible.
Also I think that players with controllers (joysticks, call it whatever) do not have "relative mode" at all but they do not complain.
And guess what next desired improvement will be? Next genius will ask for "assigning same button to different bindings" and this is because he will need to turn faoff and relative mode at the same time.
So I think that actual point is: people do not want to learn to fly.