Not the only weapon. Soul Eater is a unique weapon as well. Note that he is The Cloaked Schemer, and Roxas was the Key to Destiny, and he had two keyblades on his stone.
I too agree that Zexion isn't Riku's nobody. I even disagree on Riku having a noboy in the first place. :\
Also. In the begging, after you finish off Axel for the last time, Axel tells Roxas "See you around in the next life." Roxas replies "yeah, in our next life." Axel finishes off the conversatoin with "silly, because you have a next life." and he fades away.
So, with that said, Ienzo probably gave speical birth to Zexion. He seemed to have a different power than other members of Organization XIII, but weaponless in CoM. Powers similar to Namine's. All other Organizaton members were talented with their weapons and could manipulate elements easily. But Zexion didnt' necessarily have an element to govern, like Roxas and Namine. Which brings me to believe he had a special birth like them as well.
But then the part that mixes in with inner-demon's theory is that Ienzo is a keyblade weilder, because Sora and Kairi both have keyblaes. And I am assuming at this point that "special birthed" nobodies' other halves are keyblade wielders, because there is probably light still in their other halves that makes the keyblade chose their other personas. >.>
Eh, confusing though, considering the fact that Ienzo is most likely a Heartless like Xehanort as well. And Heartless are able to weild the keyblade to unlock hearts. Displayed by Xehanort in the first Kingdom Hearts.