After all this time, I think it was all the more important for them to be in KH III, because they're friends who have been on a long journey together, and they have to see it through after all these years. Literally, years.
To me, Sora being friends with Donald and Goofy is a thing, and it wouldn't feel right if that was taken out.
Also, if it wasn't for them we wouldn't have iconic Goofy dies scene from KH2 :p
Haha, I was just thinking about that scene.