I can see where people would want to not invert their y-axis, up being up makes sense. But when I first started gaming, I played alot of flight simulators, one of the old ace combat games was a favorite among my middle school friends and I have inverted ever since. I have done both, but I personally think inverting is more intuitive, like what ThatJagoGuy said, It feels like your pivoting a camera from behind over a fulcrum.
Although, It is very difficult to do the minor directions with inverted (you know, up and to the right, or down and to the left, etc.) But the only game that being able to easily look in minor directions was important was online Halo. So other than exteemly competitive online FPS's I always invert.