For me it's a matter of quickly referencing material. For whatever reason, I seem to recall where things are roughly by the thickness of the stack of pages. The percentage bar on the Kindle doesn't replicate that. And flipping between pages on a device just takes too long. So, I have a hard time reading any sort of reference material on a Kindle.
It's never as fast and document search on e-readers seems to be straight word match rather than any form of intelligent indexing. If I don't recall where something is, it's almost always faster to use the book's index than it is to naively search through the doc, in my experience.
