I'm gonna analyze your points here:
"If use colors from the bg, circles might blend in making the map hard to read"
Experienced mappers know how to choose them without actually blending to the background, also who's gonna play with bright background/background dim as < 10% ('Cause that would actually blend like you said) ?
"If use colors contrast from the bg, it's ugly"
You also mention gimmicks here so this is kinda pointless. Contrast with background only use if it's intentional in concept, otherwise you'll just use color as the same as background.
"90% players turn off custom skins"
This is just like saying custom hitsounds should not be used because it will be turned off anyway. Custom combo color is one of some elements that will make the soul of the map. People turn it off, yes, but can you guarantee that they wouldn't mind if the map has only default colors?
This might be inaccurate, but I think you got my point here.