I tend to play between either 90 - 100 % dim, but personally ~ I like being able to see the backgrounds in the song selection screen..... without it my osu would almost permanently be black, which isn't really what I want
I would say that if this was implemented it would be better as either a tick box option or a separate slider entirely...
I do definitly understand your point with some of the leaderboards and how skins could clash, admittedly at times it has irritated me a little bit ~ but I wouldn't want to be forced to have a blackened screen at song select simply because I want to play like that ingame