There's no logic I guess I could understand the body thing, and Ansem being able to go back without leaving anything, being a heartless...but time travel again to reunite the darknesses for KH3? They're all going back to their respective timelines after 3DS to basically be destroyed, so you would have to collect them like the first time, changing time by pretty much making 3DS not happen, breaking a rule since time will be altered o.O And even if YMX remembers, the others won't...