If you took Kimahri down Rikku's path, and took him far enough to get Steal and Use, certainly, there's an argument there that Rikku's initial advantages (Stealing and using) have been overtaken by Kimahri. However, her real strength is, as a matter of fact, her surprisingly high HP and strength values. Her first "circle" of nodes boasts many nice HP boosts, and she does get a fair amount of strength, given her appearance.

Actually, if anything, Yuna was the character I really struggled using until endgame. I didn't really need to heal much. Speaking of her, I hardly ever used Aeons either.