It's just what I like. Everytime i've had to deal with horizontal dropouts it's a pain. I like the comfort of knowing my wheel is straight and secure. Even if for some reason I don't lock my skewer down tight enough, it'll still stay in.

I might be more inclined to run an IGH if I could use standard STI shifters. I like road bikes, I just want to be able to run studs and disc brakes really.