this will also change when more and more people are opting ebook version to the printed copy. as of now the printed copy sells more compared to ebook format AFAIK and the ebook format is catching up fast for the many convenience stated in the posts here. We can expect a change soon as many people are getting ebooks and raising their concerns.
this could also change when some really nice product(device or software) to read ebooks comes to the market, I dream of a ebook reader where I can annotate, mark and make notes using a stylus like device. while many of these features are available in the ebook readers today they don't give the same feel as using a printed copy, for example (making handwritten notes)