Actually there's one quest in limsa for the main story that forces you to tank or you'll fail it everytime. For Muraders

Basically without spoilers, you get into a fight where you and a group of members are attacking a boss. Then the boss summons a whole bunch of adds, and some of them go after the healer. If a healer or to many members of your party dies you fail the quest. What the fight forces you to do is attack the mobs attacking the healer and grab hate off of them through overpower etc etc.