To keep up appearances perhaps?
The thing is though that they could have avoided this if they didn't cram everything into the last quarter of the game. More build up for different characters spread out over the entire length of the game would have done wonders to prevent the whole "rushed" feeling. Like, they started amiably with having the Twilight Town story focus on Roxas, that's something they should have kept going throughout the entire game and include other characters as well.
Saving Aqua (and by extension Ventus) at the halfway point of the game instead of right before the finale would have allowed for those two to recuperate and bring up a few extra scenes with them to better build up everything for example.
It's as so often with KH not the content itself that's problematic (except Kairi's "arc", that is heaps of problematic), but the way it is paced and presented. When looking at how KH III deals with Ienzo and Ansem the Wise one can see that compelling character arcs with enough attention are possible.
Yea, it was about soul and body being separated and stuff.
It's like the old days with Deep Dive and the first glances at MX and TAV in the KH 2 secret ending again, lol.