Tons would have changed. The game is dragged down because Xion is the only character that is given plot focus to the detriment of everything else in the game. A bunch of character's characterizations are messed with, their arcs derailed, their agency and development absent all in service of her plot and arc in the game. It would be a very different game if she wasn't in it at all. Like, make no mistake, Xion is the actual protagonist in Days.
The word y'all are looking for is not "pointless", it's "unnecessary".
Sorry, I haven't play 358/2 days in a while so i forgot most of the plot, I just remember the ending.