So I'm a good ways into ranking up my chocobo. Rank 7 currently. 21 pts in Defense and 6 in Healing. I keep him in free stance. His defender attacks spike his aggro but it doesn't hold for very long (I am a bard after all). I did notice though... when he pops regen on me, his aggro see-saws with mine and he does take a few hits while the mob switches targets. He doesn't pull off me for more than a hit at a time.
I do however think that when he gets cure, he may be able to cure, regen, and emnity strike just enough to take two hits at a time before I overtake him. Or even!!!! Maybe quelling strikes will allow him to tank at rank 10??
Edit:
Got to Rank 8. Now I have Choco Cure. He is able to solo lvl 49 enemies now. I auto attack tag one and let him kill it while I kill another. Has sped up farming mats.