Some games also allow you to invert horrizontal. It can be there if people want it, but I personally don't see why. Still, inclusive additions can be nice if there are people who do use it. But I won't make a fuss about horrizontal inverting.
I'd have to agree, it comes down to personal preference. Whether it's used on a large scale or not, i still think inversion of mouse aim (both horizontal and vertical) should be there, just so people have the option should it make it more comfortable to them. That and, the more comfortable people are playing the game, the more likely they are to enjoy/endorse it :