I'm still not sure whether to call these things "covers" or use the internet generated term: "controller condoms".
Personally, I won't be buying any of these until I can get red, blue, yellow, green, in that order from 1st-4th players, as that is the classic SSB color selection. I had that on the N64 and part of me died inside when I realized I would have to paint my GC controllers if I wanted it on the GC.
It just makes sense when you have to swap controllers when you have more than 4 people gathered around a TV for SSB. You can immediately tell on sight which controller is which so it speeds the process along.