Even if it doesn't do straight nines, it's enough. Not everyone is going to slay the aforementioned luminescent multicoloured orb of destruction. Eiko just is better than Garnet, factoring in that most of the Aeons Eidolons suck anyway. Eiko is a better White Mage than Garnet will be, and some of her summons are more useful than Garnet's anyway.

Edit: Holy usually does at least over 6,000 damage for me. But, I'm not level maxed though. Still, that's quite enough damage.