Don't ever use Mage's Ballad by yourself.
When it's active, you do 15% less damage.
However, it speeds up the MP regeneration of you and your party.

For example, if a healer is low on MP, they might ask you to sing Ballad. You do NOT have to keep it on until you run out of MP, though, just sing for a few seconds, until the healer's MP is back to a high level, then turn it off.