Ranking in low diffs is stupid, many friends I have used to rank on easy/medium maps (they were newbies) and played easy maps like 80 times to get a top 40, while me or any other somewhat skilled player got top 10+ on the first try, and as you can see, it's pretty normal that on easy/medium/even hard maps the first scores use pretty much every mod they can. On multiplayer the same would happen, if you want to compete with other players, you can just try to get higher scores than them on specific maps playing on single player.
Plus, for example, I see a noobs room playing an easy/medium map, it'd be pretty easy to raise my win ratio there, since I'd get easily an S with 99% acc if not an SS (as any player than can easily pass lunatic maps would do), while the rest of the players would be playing maps they maybe consider hard for them, therefore they will never win and it wouldn't be fair.
This would be a good idea if applied to, let's say, organized games (like tournaments) or special rooms that require the players to over rank over, let's say, 2k, or something to avoid any kind of abuse. Sadly, this wouldn't work on "rank lower than X", since I already see people using multis to get free multiplayer score or whatever we should call it. Sadly, again, applying something like this to low ranks/low skilled played is pretty much impossible, since the same will happen as what happens on easy maps, high skilled players will easily take the first places and you can do nothing to stop them.