Probably you get so owned when fighting Riku it becomes like the end of Crisis Core where you take on the army and have to die, something like that.
Forgive me for my lack of Crisis Core knowledge, but I only watched my friend play it, and the ending no less.
Oh, and I doubt Roxas would want to fight Xion because he seems particularly close to him. Then again, Sora was close to Riku and they fought, so just maybe. But Xion would get severely outclassed, she's (probably) younger than Roxas, well, that's what I think, even otherwise they would probably be as old as each other, in terms of formation, and its 2 keyblades on 1. Sora had the advantage because of much longer time spent wielding the keyblades and 2 adventures to 1 (KH1 and CoM as compared to 358/2). So Xion would probably get raped in 2 hits or so. I can just imagine it: 1 combo and Xion collapses.