I don't understand this line of reasoning. This is already represented by the character not gaining full progression in each class; that is to say, when the party hits 10th level, Joe Singleclass is a Ftr/10, but Jonny the Munchkin is, say, Rog3/Ftr5/Clr2, not Rog10/Ftr10/Clr10.
To put it a...