There are a lot of factors to consider. Does the tank dive first? How deep do they dive? Do they have a healer? Is their BS invincibility LB up?

It also depends on whether or not you have the skills to CC the tank when they hit low health to confirm the kill since you can't necessarily depend on your teammates to do it. If you're something like a Bard or Black Mage getting an early kill on a tank is a lot easier.

Sometimes you can burst a tank down, sometimes you can't. It just depends. A lot of times it can be worth it to take them out if you can since it will stop your brain-dead teammates from wasting time on them.