The Limit Break bar usually resets whenever you pull a boss, so if you're in a desperate situation where odds are you've already lost then there's no reason not to use it. I have been in a few battles where using it actually prevented a wipe and let us win, however those are very rare.

To be honest though, the healers shouldn't be the ones to use the limit break and it should instead be used by a Bard if they're still alive. Since a healer isn't stuck in animation, it allows them to keep on healing and hopefully prevent people from instantly dying again.