This should be a pro cap argument, The more high level players there will be, the harder it will be for lower level player or new players to find a group of player within the dungeon level range if they don't want high levels to come help them.
Well, that's more-or-less covered already. Maybe I can rephrase it a little. One sec.

- When there are no caps, using high level characters in a low level dungeon will become the norm. Ironically, it will become hardest to enter a dungeon when you are at the appropriate level, because noone will want to risk failure - especially once most players have a high level job
Better?