honestly every character has some type of importance to the series, big or small. Not all of them are developed the best a lot of them could use some touch ups but to take them out of the game all together would change the story line quite a bit. (for example if Xion wasn't their, Roxas would have just did his everyday missions and would have never met twilight town Roxas, and then Sora would have had to wake up some other way. Also, yeah not all of the organization members have a huge part but I guess some of them are just there for just the fact they can, same with some of the ff characters) But if I had to pick I would say the characters from the world ends with you, it was just kind of random to bring them in, they could have just made new characters all together but to bring them in KH probably promoted them so that why they did that rather than make new one.