Healers use their limit break all the time when things go wrong to revive the party.

Tanks don't use their one much but a really good tank can find some perfect split second moments to use it to prevent a wipe and sometimes it can be used preemptively before a healer limit break would be needed. You can use it in howling eye extreme when the party kills the shield plume and you have the option to use it in jade stoa as an alternative to bothering with the DPS check.

The tank limit break would be better if it applied a shield that absorbs damage, maybe worth 30%, 60% then 150% of maximum health with a much longer duration, at least 30 seconds. The biggest problem with it right now is that a buff that expires after a few seconds feels useless, but if it gave the party a period to ignore mechanics completely for 30 seconds because they have a really strong shield, it would not always feel like such a dent on overall DPS and it would prevent the DPS from having to interrupt their rotation to use it too.