Actually, she was only left out because her voice actress wasn't available. They decided to use Yuffie instead, who didn't have a designated voice actress at the time.
Oh... Well, that's a bummer. They didn't use Tidus's and Wakka's, though, so... Eh, it all worked out for the... well, convenience, at least, with KHII's YRP cameo.
Think of the subtitle dilemma, though (unless it's someone out of the loop on one or both of the characters). Besides, scarf wings gets some amount of points.