BLM can help groups plow through trash very quickly. That's an advantage for having a BLM in the party. They're dmg is lower vs bosses (I guess because they have to move from AoEs instead of timing their spells right) which puts them at a disadvantage. I dunno, seems balanced to me. You can take a MNK instead of a BLM. MNKs might make the party move slower through trash because there are no AoEs, but they make up for that disadvantage at the bosses because that's where they shine. No job should shine at EVERY POSSIBLE situation.

BLM speeds the party through trash, but has slower boss kills.
Single target jobs suffer from slow trash kills but wreck bosses.

What's not balanced?