Limit Break Caster or DPS First or Last Boss

[Canadane]

Using the LB against mobs is generally a waste in my opinion, it doesn't kill them in one hit, and if the party is worried about surviving a giant pull with the use of an LB, tank LB might do a better job, keeps the mobs from doing too much damage for a while...

Killing them faster with a ranged LB will keep them from doing more damage than letting them live with a tank LB preventing some damage over time.
It's he same reason dps is priority for all classes in the games meta as it is. You simply prevent damage by killing them faster.