In WoW raids an off tank is needed to pull the adds together so they don't kill the raid. There are not many engagements in FFXIV that do require a second tank for this though. I thought there was a fight that had two bosses, but I think I was just thinking of the Romeo and Juliet Opera encounter of BC Kharazan.

In XIV there does seem little point to have two tanks, it seems more like wasted DPS potential in this game.