Leviathan EX is a great example of a fight where the caster LB is preferred over melee, especially if you have level 3 because positioned right you can not only hit spumes, but you hit leviathan and his tail, so you do double damage to the boss which is usually more then the single target could have pulled off and the added benefit of killing those glowing balls of awfulnessness. Unfortunately this also can cause phase skipping, but then again so can over gearing for any of the primal fights :3