Tank limit break

[Windwalker]

Only seen it used in Alexander Prime

[HyoMinPark]

Only seen it used in Alexander Prime

It’s used in both Ultimate fights—UCoB’s Seventh Umbral doesn’t require it if you have a WHM that can handle the healing (though it’s recommended for progression/if you have SCH/AST), and it is required for Ultima’s initial appearance during the LB3 intermission, his Ultima cast after Suppression, and you can opt to use Tank LB1 for Aetheric Boom if your healers want you to.

dps never use their Limit Breaks either except the very end of a given fight. That's not very impactful either, nevermind turning the tide of battle. At least Tank LB3 lets your party survive Boss LB's that would otherwise cause wipes, like Alexander, and Omega.

Tank LB3 can also be useful for progression to allow the party to see more mechanics. Some groups in UCoB opt to Tank LB Heavensfall just to see more of the mechanics.