As others have said BLM/SMN LB is an AoE (I haven't used it so I can't comment on the damage) therefore it should do less damage to one target or you would never see a DRG or MNK using theirs, this way if there is one target use a melee LB, on multiple targets use a mage LB this idea of making each LB situational allows everyone to unleash hell every so often.