Not only are these books totally unnecessary - FH jumps forward in time between books for a *reason* - they are clearly just milking the Dune brand for all its worth.
I found a Jan 2000 online interview with KJA and BH which is illuminating. I've included the whole question and answer, to preserve context. Emphasis is mine.
You don't do the grand finale and then add a few more books.LR: What about Dune 7?
KJA: Well, we have a full outline for Dune 7. We know where Frank Herbert was going but we've been talking to a lot of the fans and we've been talking to each other and we think that the next big story that needs to be told is the Butlerian Jihad -- the war against the thinking machines 10,000 years ago. It tells the original split between the families of Atreides and Harkonnen.
BH: Isn't that timely, also? Machines against humans? Artificial intelligence we're concerned about now.
KJA: So we have the outline for Dune 7 and we know where Frank Herbert is going and where he intended to end the story, but we find that there's a lot of things we need to set up before that. And also it seems kind of fitting that Dune 7 should be the grand finale. And you don't do the grand finale and then add a few more books. But, we have the notes. People should be able to have Dune books from now on.
BH: Every year for a while, but we're going to maintain the quality or else we won't write one.
So don't, BH and KJA. Just don't.
HBJ