Its MP cost is a gain over Dark Arts if it pops, and its also the only cooldown you have available in between Rampart casts, so unless you want to take 70s-worth of completely unmitigated auto attacks/cleaves/raid aoes, use it more or less on cooldown. Line it up with any cast the boss does, time it so it catches two AAs, etc.

When casting it on your co-tank, its much weaker and pops very easily, so it should be cake to time properly.