>>12 He had to wake up because he's the main character. But really, Xion could have been saved if it hadn't been for Namine's meddling. Riku and Riku Replica were perfectly happy as separate beings, no memory muddling going on between them. If Namine had only given Xion a copy of Sora's memories, Xion could have lived. But no, Xion had to have Sora's real memories, and so she had to be absorbed back into Sora so he could wake up. :/ kind of lame.
Since Dream Drop Distance takes place in a dream world, we might see Xion again as a dream.