No offense, but whether Purification is useful for AoE in dungeons or not is besides the point... cause you shouldn't have an ability that is only good in a dungeon. At least not in an expansion with only 5 abilities that are supposed to address the jobs disadvantages and/or just make the jobs feel fresh.

Purification could actually have been a very good addition to monk if it had nothing to do with Chakra or worked in another way... but why would I gather 5 stacks of Chakra only to use it on TP, when you wanna have a BRD/MCH give you that anyways? If you have a NIN in your party, you would just use his Goad, and he would need TP from BRD/MCH around the same time anyways, and even though DRGs last longer, it wouldn't make a huge difference for the BRD/MCH to use their TP regen earlier anyways.

In raids, which they should have focused most on, it's totally useless IMO. Maybe if there are some AoE phase in the raid where you need some extra burst TP, but that's a very limited use.

Except for AoE, you need the most TP when there's not downtime, but you can't use Meditation during that scenario unless you wanna totally screw over your DPS.