I don't really know anything about skating. In gymnastics the real reason mainly seems to be because it just is that way and we're a fickle bunch from what we can see.

Half kidding. I don't mind the look of long sleeve leos, but I don't think it necessarily is more aesthetically pleasing. Personally I think the cut of some sleeveless leos is more flattering, but I guess the long sleeve does kind of look more formal and just seems more "right." I don't think of the line as that much nicer but I'm guess I'm just not really into that whole game of "will this pattern emphasize bent arms more" etc. I HATE it when the upper body/arms of a long sleeve leo are baggy though, it looks horrible to me. I'd much rather see sleeveless.
I'm seeing more and more sleeveless here, it was pretty typical for level 4 teams, now I see gyms with their entire compulsory program in sleeveless. Now I can't think off the top of my head of any of the bigger gyms having the optional programs go completely sleeveless, but I can think of a couple that get a "warm up" leo and then wear it for some entire meets but they still have a long sleeve leo and wear that at others. That seems to be the more common situation right now.