If you actually think about it, Sora couldn't have saved Kairi without Riku. For it was Riko who found Kairi's body after plunging into the depths of Ansem. The only "evil" Riku was Ansem controlling him.

Besides, Riku was a damn cool boss, and if Sora didn't have all those Curaga spells and potions, he would have been owned by the power of darkness.