Xehanort needed brainless clones of himself because he didn't trust anyone besides himself (friendship is for chumps). But he did trust Sora, who has proven that he will follow his heart no matter what, so Xehanort uses that in both DDD and 3. However, I think that this dynamic could have been perfected by taking advantage of the fact that sora has given his essence to six other people over the series, effectively making seven lights. (Sora, Roxas, Xion, Ventus, Namine, Vanitas, Data Sora)
I imagine the lights, and traitors in the darknesses ruining Xehanort's keyblade war. But he pulls Sora into the dream realm and merges with his replicas. Sora is too weak to beat him, so the six that have his essence could give back the parts of him they don't need anymore, effectively making the clash of 7 vs 13 into a one on one between the only two people Xehanort can rely on. That would have really felt like a massive chess move from Xehanort, and made all the Soras over the series less of a meme and into a vital part of his plan
