Roxas was a nobody, who just like the rest of the nobodies in the org, who wanted to be real, or to exist. Saix said that each heartless slain by the keyblade adds to kingdom hearts (as long as they collect the hearts). Once kingdom hearts is full of hearts, they can use the power to make themselves "real" (like full beings). Roxas had the power of the keyblade right? Why didn't the org just summon loads of heartless, ask Roxas to kill them, collect the hearts, and repeat the process again. Axel would love that, and so would Roxas and the others. What was the point of using Sora? The instant Roxas joined the org, why didn't Xemnas order that to be done? the could quickly build up kingdom hearts like that, they could even weaken the huge heartless, so that one swing of the keyblade would finish it off. This seems like a huge plot hole you know. The instant Roxas joined the org why wasn't that done? Instead Roxas spent a good deal of time with the org (like a year) and then went to find his "other" to find out why the keyblade choose him. Sora managed to fill kingdom hearts within a short amount of time. A year would have been more then enough for kingdom hearts to fill if Roxas just killed the heartless that the org summoned.