I was doing Brayflox's Longstop last night, and during the boss fight I used Mage's Ballad when the WHM was at less than 50% MP. After our second wipe he said "btw the Mage's Ballad is killing me because [....] AoE stacks and so it is hurting both us". I can't remember the exact words, hence the [...]. For anyone unfamiliar with the boss fight, it chucks an aoe poison glob at random party members.
Was the WHM talking about the poison "stacking" on both of us, making him have to use an extra Esuna on both of us since I have to be close to him for the Mage's Ballad to work?
Or was he referring to some other reason, or game mechanic, that you know of that I shouldn't be using Mage's Ballad?