I didn't mind Xion as a character, but I will admit that she perhaps did seem like a bit of a last minute thing, supported by the fact that 358/2 days was created very quickly. Ultimately though, I think that she was a good character and was expanded upon well, though she did take away from the other Organization members possibly having a bit more of a backstory and possibly a bigger role.
I think that she probably was needed though, otherwise the storyline would be bit boring. I mean, who would really want to watch two people sit on a clocktower everyday and eat icecream? I mean, one more person, especially a female added a bit more to the dynamic of the trio.
I don't think that 358/2 days would have been able to be completed as well without Xion in there, and only Roxas and Axel, so while I will admit that she was kinda random and out of no where, the game wouldn't have really worked without her presence.