Only one thing comes to mind whenever I read that someone wants a specific Poke Ball in the GTS: part of the newer breeding mechanics make it so that a Pokémon hatched from an egg will have whatever Poke Ball the mother had. I think some people may be just a little annoyed that, after years of having them get a regular Poke Ball, it suddenly changed so that the mother's ball is what the baby has. If not that, it's probably just a personal preference for some people have one kind of Poke Ball over the others.