Tanks are primarily used in interrupting the opponents from effectively healing or casting.

Don't attempt to get kills (You can try, but just don't waste your time)

When a node spawns, interrupt enemy Black Mages, Bards and Summoners. (To help your team secure more damage and buffs)
When there's no node, interrupt enemy Healers. Especially Scholars... Bard's can poke White Mages for days, you can stop Lustrates!

Target melle dps when they rush your team and you're falling back to get heals.

That's about it *shrug*