The issue is that the Devs are damned if they and damned if they dont. If they allow mix level players, you have power leveling running rampant (anyone who's played PL knows the issues this causes at the level cap). If they don't allow mixed levels, then you get this complaint.

Also, sprinkle in a little of "what level do we make the enemy?" when you have a level 28 and a 13...20-21? Then you can still power level. I'd rather have no PL and have the game be tougher than have set level caps and PLing all over.