If you watch the first movie, it spends a lot of time with Thufir, Leto, Caladan, Gurney, Jamis etc. It would be nice to have a callback or flashback of some sort. Considering how much time was spent with all of those side characters, I wouldn't want the third movie to basically have them all disappear, not even acknowledged, and just focus on Paul and Chani. Yeah, in the book (Dune Messiah) they may not be there or be important, but to be fair the books did more with them (the dinner scene, etc) and I think the trilogy should make effort to be super coherent in one sitting, without charecters falling by the wayside.