I would like to add my voice (for what it's worth) to those wishing to let the user override the skin. The OSU! default skin has excellent contrast, and I can easily see where the cursor is, whether it's on a slider ball, against any of the circle colours, or on the spinner. The same can not be said for all custom skins, which we must use to gain rank.
Examples of beatmaps that I like but are infuriating to play:
http://osu.ppy.sh/s/9533 - the custom skin is nigh impossible to tell apart from the background.
http://osu.ppy.sh/s/9651 (and anything with the Lucky Star skin) - the spinner is almost the same colour as the cursor. No good.
http://osu.ppy.sh/s/8495 - similar problem; the spinner sucks. The circles aren't great either, but it's playable.
I'm not even colourblind (far from it, my vision is better than average). I'm sure that the colourblind and blind-aspected players can come up with many more examples of beatmaps made needlessly difficult solely because of the skin.