Oh jeez these never die.
*boosts some post count*
And Kairi doesn't? Did you play KH2?
KH2 Kairi hardly fixed any problems on her own, at least not intentionally. Her very existence was another something for Sora and Riku to worry and get hurt over. You wanna tell me Xion existing made the mess? Kairi being kidnapped. I fail to see how her staying on the islands would've ruined KH2's plot line, even with BBS implications in mind. The most she did was write the letter and send it out to sea, but let's face it - Sora most likely could've pulled whatever Deus Ex Machina they wanted him to and got them back home, or Riku for that matter. It's nice Kairi got some semblance of importance in that way but seeing how five minutes earlier Sora promised to see her every day (a promise the FM+ gave a glorious SoNami twist to, mind) and was then content on staying with Riku in the Darkness, the impact is lessened.
Xion has a problem with her own issues. Riku would've most likely eventually dragged her by the hair like he did for Roxas, making the value of her decision questionable at best, especially if you didn't like the girl to begin with.
However, for whatever reasons of whatever quality, Riku DID give Xion the chance to reach her own decision, thus letting her paint the entire mess in a glorious shade of self sacrifice.
Both girls could've been written better, but they're in opposite ends of the spectrum.
Kairi was a plot device in the sense that left very little place for character growth, development and insight, in great part because she was so passive about it. It's true that she wanted to find Sora but it's her being kidnapped by Axel and Saix and then saved by Riku and Namine that even let her come face to face with Sora, leaving her intentions to be blown away by the hollow winds until they actually have her do something about it, and not go along with the flow.
Xion was a plot device in an already predetermined story course. We all knew what would happen and with BBS leaving little to do with the rest of the members (and some members, seems to me, had nothing to have done with in the first place [ironically I enjoyed Demyx and Luxord the most in Days]) we got Xion development and insight shoved down our throats almost as badly as they can get when trying to convince me SoKai exists without the plot forcing it. The end result was, however, that Xion was painted as one who tends to take the initiative, even if her childish (at least at first) nature and lack of knowledge about her own situation made her more run around all over the place in search of answers - and then she up and done whatever she could, even when her hands were forced, as cruel as it might seem to some of us.
tl;dr Xion had the chance to actually DO something whereas Kairi didn't, for all her intentions.