1. You're leveling a DPS at, level 59 for example.

2. Suitable dungeons are all waiting for tanks or healers, so you wait a long time.

3. You have a level 90 tank or healer, but it doesn't need XP, so you don't run dungeons on it.

4. How about: You run a level 59 dungeon on one of your 90 tanks or healers, and reassign the XP you would have received in the dungeon to your level 59 DPS alt job as if you had run it with that job.

This feature would provide players with an incentive to fill lower level tank and healer slots, while leveling their alt DPS slots a bit faster.

I suppose there might be some issues with this, so let me know.