I think that the entire Nobody idea didn't work out well... And I KNOW that the whole Ansem/Xeahnort thing did not work at all.
Ansem in Kingdom Hearts was not a Heartless. Proof of this is he is not listed in the Heartless part of the journal. He did not show any real Hearless qualities, other than a necklace with the symbol on it, until he did God-knows-what to himself in the final battle. Yeah, I really hate that battle. So anyway, Ansem (Alright, Xeahnort's Heartless) was originally not a Heartless.
This was only established in 2. Ansem also referred to himself as Ansem, which is why KH2 had to do a massive (And in my opinion, pointless) retcon. After all, why would Xeahnort's Heartless (And Xeahnort himself) call himself Ansem? It makes no sense. Xemnas suffers from much of the same problem.
Yes, that "Ansem" was Xehanort's Heartless. Mickey explains it when the trio reunites with him at Hollow Bastion, and it's also explained more fully in the Secret Ansem Reports. Years before the first game happened, the real Ansem the Wise found Xehanort barely alive with no memories, and took him in. Xehanort became one of his apprentices and together he, Ansem, and the other five apprentices worked to try and solve the mysteries of the heart. It was when they started doing tests on a human subject (Xehanort, who hoped to regain his lost memories) that everything started going wrong. Xehanort began to tamper with the darkness and lost himself to it. He also led the other five to follow him. They sealed Ansem in the Realm of Darkness, and sometime later, they themselves lost their hearts. Once that happened, their former selves became Heartless and their Nobodies were Xemnas, Xigbar, Xaldin, Vexen, Lexaeus, and Zexion.
Now, due to the unique circumstances in which Xehanort lost his heart (Ansem believed he willingly gave it up) his Heartless possessed a will of its own and remembered who it was. Also, not surprisingly, his Nobody was a powerful being who could retain a human appearance. While Xemnas was off doing his thing, Xehanort's Heartless was unleashing his little army and causing the havoc that would lead to KH1.
Why did they call themselves Ansem? We don't really know for sure, all we know is that at some point Xehanort started doing things in his master's name, including writing the Ansem Reports from KH1. After he sealed Ansem in the Realm of Darkness, he took on Ansem's identity.
The reason why Ansem wanted darkness to flood the realm of light is is identical to the reason the Disney villains wanted to do it. For power.
Yeah, and I believe Xemnas has similar motives. I'll get to that in a minute.
As for the Nobody wants hearts thing, that is simply because KH2 did not explain the motives of the Organization well. They don't have emotions, yet they can desire, which is an emotion. Plus, Saix (In battle), Demyx (In the Underworld, AND before you kill him), Axel (Obvious), and Roxas (Really obvious) all directly contradict this.
I have a theory that the Nobodies can feel, but barely. Their hearts still exist, whether in the form of Heartless or suspended in darkness. Roxas' heart, of course, exists in the form of Sora. I think that he and Namine have the strongest sense of emotion, because their Other exist and they feed off of that. Axel could also feel stronger than most because of his friendship with Roxas, and I think they formed a connection that caused him to indirectly feed off of Roxas' emotions, which came from Sora. That's why they both made him feel like he had a heart.
Their bodies allow them to retain human form. Their souls allow them to keep their personalities. I think they still can sense their hearts, but with those hearts no longer united with body and soul it makes it difficult for Nobodies to feel. But it's not impossible, which is why you have examples like Axel and Roxas' friendship, Saix' Berserk state, Demyx' fear, and Namine's guilt.
Then, why do the Nobodies need so many hearts? It's because the story team needed an excuse for them to want to get hearts. There is no reason why they couldn't just choose any old random heart (Unless there is an example, in which case, state it.). We saw the Dusks trying to grab the hearts falling from Kingdom Hearts. We never saw a Dusk grab a heart, so it isn't certain that they could or couldn't rejoin with a random heart.
I believe that Xemnas was lying like a rug when he told the other Nobodies how they could regain hearts. Either way, he really didn't care about becoming whole except so that he could have more power. Once he became complete, he could continue to harness the power of those captured hearts. Whether or not the other Nobodies became whole wasn't his problem. He just recognized the power in them and decided to use it to his advantage. Look at how he treats Roxas. The descriptions we've gotten of 358/2 Days says that Roxas MUST collect hearts every single day. It's all he's ever allowed to do. Xemnas obviously saw him as a useful tool, and nothing more.
My point is that I don't think Xemnas was really after what the rest of the Organization claimed they were after. He told them it would make them whole and they believed it. I don't know why they thought it would work, but they did, and so they followed his orders, and paid the price later on.
