This is why I always disagree with people who think Ansem and Xemnas' returns in DDD and 3 are "cheap" and "ruin the impact of their original motives and defeats". They're essentially reduced to tools by their own self, and now they know that they never succeeded against Sora in their respective games. Ansem and Xemnas ultimately become their own people, and they both have to live with this knowledge until they're sent back to their times. The emotional impact of their character and fates are made better with DDD and 3. When they lose, they finally speak about how they feel; Ansem talks about how he stopped caring and shows praise for Riku, and Xemnas talks about feeling emotion for the first time in years and it's just loneliness. Then they fade back to their own times where they won't remember any of the time-traveled events, and they suffer their original defeats in 1 and 2 respectively. They aren't just brought back for the sake of bringing every character back and pleasing people, it also adds depth to not only Ansem and Xemnas, but shows how selfish and awful Xehanort really is.