A caster LB1 on just 2 targets does more damage than a melee LB1.
A caster LB1 on 4 targets does more damage than a melee LB2.
A caster LB1 on 7 targets does more damage than a melee LB3.
Caster should use it early on a trash pull, then again on a late trash pull. If the bar is full before the boss, it should only be saved if the boss has three or more simultaneous adds.
Melee DDs shouldn't LB except on primals and in non-alliance raids.
(For comparison, in a full party:
A caster LB2 on 4 targets does more damage than a melee LB3.
A caster LB3 on 2 targets does more damage than a melee LB3.)