Kimahri is useful for one very important purpose: fighting Biran and Yenke, both of whom hold Lv. 3 Key Spheres for the stealing. Train him as a Thief, and he will be able to perform Luck on himself about ten times and then steal from Biran 5 times and Yenke 5 times. Combine this with your ability to have a Master Thief armor at this point, and you can steal 20 Lv. 3 Key Spheres, which is enough to open every single Lv. 3 Lock on the sphere grid (if not enough then close to enough). And for this reason, he is more than worth the training, as you will have to either fight an endless supply of Defender Zs or wait until you can have access to the Omega Ruins to bribe a Demonolith. I personally don't like doing either of those.