Bob, I think we are actually in agreement. I admit that the depth of focus is extremely shallow (small) when compared to the depth of field. While I am not an optical engineer - I have always believed this to be a function short projection distances involved (not unlike the shallow depth of field experienced in macro work.)
Ron Wisner wrote of the flat field lens which I found to be informative and enlightening.
The point where we seem to disagree is whether the depth of focus is actually significant enough to resolve for a lack of flatness in the film plane. It has been my experience that it is (given my equipment and processes.) If that has not been your experience, I would certainly not try to invalidate that. I, too, switched to glass carriers because I have a broader range of exposure choices since I am easily bored with long exposures.
Good discussion. It is always valuable to learn of others' experiences.