I've been having this issue while framing Alexander..

Situation 1:
Me: <Mage's Ballad up>
Healer: I don't need MP

Situation 2:
(After a wipe)
Healer: Bard!
Healer: No Mage's Ballad?
Me: You didn't ask for it


Even though in Situation 1 the healer had low MP. I usually sing when MP is around 10~15%

So question to healers, at what ???% do you need Mage's Ballad?