Great feature that would be cool to see expanded in future versions. A common issue I have with IF is not knowing how far along in the story I am. With a print book, you always have the weight indicator, and I enjoy seeing people implement similar features in their IF. In my story, I've numbered sections to help ground the reader just a bit. (The progress bar, unfortunately, didn't work properly in a story as long as mine, which makes sense given the alpha state.)
Absolutely, we played with the progress indicator for exactly the reason you mention -- how far along am I? But when you have branches that are asymmetrical in length then it's not straightforward to compute. With your book metaphor, instead of a linear book think about a choose your own adventure book and how hard that was to gauge how much of the book you'd read.
Totally makes sense! Perhaps there would be a way for authors to number their own pages (or parts) in some sort of customized way. It definitely wouldn't work for every story, but I could see people structuring by short "checkpoints" or something (like a save point in a video game) out of a total. But yes--definitely a complicated issue.