Giving MP to a healer not managing their existing resources is a mood, and the healer should be punished for not using their tools correctly imho. An LB shouldn't be wasted to fix that as it's not adding any real value. (We don't need healers wasting the LB to recover mana, could you imagine? xD) [I could totally see this being a thing with a bad healer where crowd control is an issue but a simple caster, ranged lb could clear it quicker]. Or even worse, on a boss where a melee lb would be preferable more...
If the healer is failing to heal or manage mp, even a full restoration won't probably mean a thing. Something else is going wrong if you're running out of mana that quickly. (Are you spamming heals(?))
As for making it more useful, here's something:
LB 1: Resurrects one target at random to full hp, removes and prevents any applicable death debuff that has or will apply on that target.
LB 2: Resurrects two targets at random to full hp, removes and prevents any applicable death debuff that has or will apply on those targets.
LB 3: Resurrects party to 100% and removes and prevents any applicable death debuffs that has or will apply on the incapacitated party.
(If you're using an extremely limited resource (the limit gauge), I'd argue that the death debuffs removal is okay and actually helps with adding some value back to the role, and in fact I could see a mechanic being tailor-made for this, where only the healers survive but bring back the party to full damage [Thus contributing to their value to the battle] (Think the Healer LB 3 equivalent to a Tank LB 3 in a fight that requires it)
In cases where some unique mechanic does not exist that makes use of that logic, it still has some conditional use if someone dies, although, still requiring someone to die lel. (Though I don't see that as bad, as the healer LB should remain conditional though given the context of it being a healer LB)
I do think LBs should contribute to some value no matter what LB you are at, I mean the tanks' LB and dps' LB do...so it makes since the healer one should also. Just like the tanks it should be conditional, where as a tank you might see them pop the LB early to save themselves in a pull if the dps aren't using it or in a fight to save the remainder of the party. It should be following something similar to that...conditional but when that condition comes about, another tool you should utilize.