Wasn't it a failed experiment to just split someone into light and darkness - hence the X-Blade was incomplete in BbS?
Also, even though Xemnas may not have been fully aware of what he was doing, he was basically assembling the pieces to transfer the darkness into the Organization to get the thirteen darknesses... which thinking about it now, makes KH2's story even worse and Xemnas (in that game) make even less sense than what I've thought before.
If you look at Xemnas strictly from a KH2 standpoint, Xehanort was an idiot that decided to turn people into nobodies for fun, then realized that being nobodies sucked and thus wanted to create Kingdom Hearts to find their hearts and become whole again. Basically, he caused the issue himself (and unleashed Ansem to cause the damage to all the worlds) and the game tries to make it tragic.
If you look at Xemnas' story with the context of the entire series... Xemnas overplayed his hand by revealing the Organization's activities to Sora, thus becoming a target. Xemnas lost all the bodies that he needed to transfer the darknesses into by having them confront Sora (when there was no actual storyline reason to do so). If Xemnas really wanted Sora to kill enough heartless to summon Kingdom Hearts, he could have just had Sora go on a wild goose chase across Disney worlds looking for the Organization and never having them show up - knowing how easily distracted Sora is, Xemnas should have known he would help the Disney characters fight heartless - hence summoning Kingdom Hearts without even risking the vessels he needed. As much as I love Xemnas, his story in CoM/KH2 is just not good.
My nitpick comes from this... it's claimed that they needed Kingdom Hearts to "hook up" the vessels and be used to transfer Xehanort's darkness into them. That can't be right since the plan was to use the darknesses to summon Kingdom Hearts. Unless Xigbar (who I believe said that they were going to hook up the vessels to Kingdom Hearts in 3D) got the plan wrong? Since it's clear Master Xehanort can do the possession thing without Kingdom Hearts.