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.
KHII's YRP cameo is blasphemous. Worst cameo I've ever seen. I would've much preferred to see Rikku in a mini walking with Kairi than some trio of poorly designed fairies (based on an awful sequel to begin with). Heck they could have used the Riku/Rikku name confusion to their advantage in some side plot...
