for the record, I think there are some great reasons to scan books from paper, mostly having to do with verifiability.
As we saw with the recent review of german epubs, many of the books being sold as e-books are in fact scanned and (poorly) OCR'd versions of the printed book. However, the publisher never gives you the page scans so you have no way of checking whether the OCR error you found is an error or not, or just have confidence that the section you are quoting somewhere is actually being cited as published, page number or formatting and all.
As for reasons you might want to scan from a book reader:
- fast (~750 "pages"/hour)
- easy (page turning becomes a button press)
- cheap (you can do it with a single consumer grade camera tied to a pole)
That said, if you're using something like the bkrpr.org book ripper here:
http://www.youtube.com/watch?v=yjRKeHPRa2k
You keep the speed, can use physical books you already own, and get the page images for ultimate verifiability, all for the addition of a second camera, a plastic piece, and turning the pages.
Whatever fits your needs.