Dungeons aren't really worth doing more than once. The loot tables are huge and random, meaning you could run it dozens of time and never get an item for yourself. Once you stop getting chains from mobs inside of them the experience gains are extremely lackluster when compared to Fates.

Grinding Fates at level 34 nets you about 8k experience, 300 gil and a chance for HQ loot every 3 minutes (or less) for just zerging stuff and spamming aoes. So yeah.. either Fates are broken or Dungeons need to reward more experience because as of now they aren't worth the effort.