This is a very bad idea from my point of view. Approach Rate setting is very personal, one reads map by reaction, someone else by it's structure, etc. A good example would be that map yyy was talking about, he thinks it needs AR 9 and lesser AR for slower parts, I think it look just great with constant AR 8.
I kind of don't feel like stating obvious things here or to list my different rhythm games playing expirience, so the point is: we players already have to deal with hellish number of differents points of view on mapping and how many objects should be there on screen on the same time, adding this feature (sure, sure, there are good uses for this too, but) would cause only more frustration, since AR is only really decided by mapper and approvers of the map and can't be changed client-wise.