As a user of FrameMaker to produce several dozen published textbooks, I might perhaps be qualified to comment.
Although I have bought a FrameMaker 10 license to keep on the upgrade path, I am still using FrameMaker 7 on Mac simply because it does everything I need it to, predictably and well, including structured. This situation will pertain until I can no longer support FrameMaker 7 Mac. I suspect I am not alone in this. As for numbers of seats, I was told anecdotally a number of years back by an Adobe employee that it was the volume license users such as Boeing that kept the product going. -- Steve
