If you send him down Rikku's path and learn to steal, you can get all the Lv. 3 Key Spheres you'll ever need cause both Biran and Yenke have them. Especially if you visit Remium Temple and use the items from the chocobo race to build an armor with that ability that ensures that you get rare items when you steal, cause then you get 2x Lv. 3 Key Spheres with every steal. Other than that I never found much use for Kimahri after the Mi'ihen Operation.