I agree. If you want to make a level for Jill, Meat Boy wont win. You will likely verify as Jill.
I also believe that it's not nearly fair for a level to be open and have people that aren't meat be unable to beat it.
But there are plenty of official levels that can't be accomplished with all or most of the characters. Take the level in the Salt Factory Dark World (I think):Black Circle. Once you get to the bottom of the screen and need to go under, I believe Meat Boy or RunMan are the only ones fast enough to get through without getting sawbladed.