My guesses: 1.) It makes more room to access the star wheel/detent for the shift drum. 2.) It will have to come off anyway if the cases are going to be split. I really wouldn't mind if Honda had kept using the upper and lower case halves like their old bikes had. It may still be a pain to open the bottom end, but at least the top end didn't have to come off most of the time. Earlier this year I had to replace a kickstart shaft(common) in a low-mileage XL175. Although the Clymer manual said the whole thing had to come apart, I was able to pull the bottom case without disturbing the head or cylinder.