I don't know how it is for ACN, but people seem to not realize that having Ballad on BLM means you can stay in your fire stance for a longer period of time which means that you have a lot less downtime where your damage is low (the time you are waiting on Blizzard ticks).
Also, you don't have to wait for your WHM's MP to be low to sing ballad. Ballad sung early can keep their MP up longer, and later in the fight when DD need TP you can Paeon for them. You can also do the reverse, as long as both DPS and Mages are getting attention at some point.