So if your friend stops playing, their character isn't taking up a slot in your party. If they come back, you can get their character back. They aren't "removed," they're just temporarily stuffed in a magical wardrobe.
Previously, you couldn't remove some abomination your friend made as a joke when they joined your game. You also can't rename characters. This helps with those things at least.
Not everyone wants to play through the game multiple times as it could take 80+ hours.