The balance issues for this would be ridiculous. If current mechanics stayed put, chocobos would be useless as tanks or healers. If chocobos were scaled to basically be perfect tanks/healers AI, then all player tanks and healers would be eliminated--and the game would be so mind-numbingly easy because all you'd have to do as DD is stand there and win. Why not just have one-shot boss kills instead? The desire to implement the idea is understandable, but it's not feasible without alienating 20%+ of the community, or making the game unrewarding.

Besides that, if they automated the game like this, what'd be the point in calling it an MMO? Players would just solo the content with their chocobos. Why not just play a console game? Then you don't have to grind the same content over and over, and you have full control over all tanks and healers and damage.