When I can easily browse my bookshelf in actual physical three dimensional space, it's easier for me to move backwards and forwards through thoughts and ideas that I have acquired from those books, thoughts that otherwise would get easily buried in digital layers of organization.
Don't get me wrong--I am pro e-book. I have many in my collection. While technology allows me to simplify many things and aggregate diverse objects to a single place, it doesn't solve or simplify how I organize books (and to me books represent hundreds of hours of invested time and education). To say that with an e-book everything is at my fingertips is somewhat misleading. The organization of a physical library still beats the organization of my books in Kindle or iBooks or any of the number of programs I've tried. And for a lot of my comics and art books, the dimensions of the paper itself is part of the final product.
Maybe I'm old fashioned, or maybe when I have touchscreen wallpaper I'll consider switching over to completely digital.