I don't think there's any advantage to a level top tube, except its aesthetic quality. To some people that's just the way a bike should look. I thought the sloping tubes looked weird and, well, "wrong" when they first appeared, but I've gotten used to them. Now a level top tube looks kind of "retro". I choose bikes for how they work, not so much how they look. I do like a sharp looking bike, but a level top tube is no longer part of that equation.