If you are actually trying to build fitness and condition the horse (think racehorse, event horse, endurance horse) you have to either body clip or keep the horse under lights to keep the coat down in winter. Otherwise, the horse will overheat before you get to the point in the workout were you're building fitness.
That's one reason, and the most legit reason, IMO to body clip.
Since I mostly pleasure/traill ride, my compromise is to blanket clip - just remove the hair on the belly, chest, neck and head and keep turnout rugs on. It makes a big difference in how quickly the horses break a sweat and makes riding in the winter more enjoyable for me and I think the horses.
It does make grooming a snap, and cooling out quick and easy. When I had multiple horses to get ready, it was a matter of taking the blankets off and either hot toweling or going over them with a soft brush - huge time saver. But that of course is more about human convenience.
I also had a touchy TB that HATED to be groomed. His skin would literally crawl even when I used a soft brush, and was miserable being curried. Body clipping him, so I could wipe him down with a hot towel instead of grooming, was a kindness.