In a way,there is no ending.Once you beat Xemnas there is no save point afterwards,so once you load your file,you never beat the game.This is for players that want to get to level 99,get ultima weapon,get secret ending,blah blah blah.Or for players that just want to play around more.There is no way to "finish" Kingdom Hearts 2,so yes,you can continue Kingdom Hearts 2 after the "ending".This feature is also for Kingdom Hearts 1(I am not sure about 358/2 days).