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.
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.
You don't do the grand finale and then add a few more books.
So don't, BH and KJA. Just don't.