Readability: Change font, size, aliasing, and spacing to your liking.
Convenience: access millions of books using search
Annotation: Text editor or some kind of application.
Cost: free
Social flags: You can still tell people what you're reading. Bookshelves are nice decoration. But I bet OLED wallpaper or paint isn't too far off.
Aesthetic: A book is a brick of paper. At least CDs are shiny.
Sentimental value: Any object can have sentimental value: clothing, sports equipment, obsolete hardware.
Rarity: If text is not online, it doesn't exist.
Single Mindedness: Distraction is always a problem; learn discipline or face the consequences.
Religion: Torah didn't switch to the printing press.