Yeah but then we'll have 2 players all alone on a single server farming x5 because the other guy is afk
To be honest, the fact that people worry so much about others getting some kind of unfair advantage like this is pretty pathetic. If some no-lifer and his friend want to stay up 24/7 while the rest of us work/sleep/socialize, so be it. I am quite content letting someone blow past me in levels since I gained mine not only by having fun, but also in a manner that didn't interrupt my normal life.
That said, the more realistic approach to fixing this issue would be to simply fix it so that this message never pops up when the server has more than 20 players. I have seen it popping up an insane amount lately, even during primetime in NA1.