I'm a voracious reader, and have an iPad, a Kindle, and shelves of (mostly read) books.
Out-of-print -- with e-books, that means it's really out of print. With a traditional book, it means you need to talk to a used book store.
Used -- you can't save money by buying your e-books used. Nor can you sell them if you decide they have no further value to you.
Unanticipated Reading -- e-books are great for unanticipated delays where you brought an e-book device, but didn't bring a book.
Outdoor use -- you can take a paperback up a mountain, onto a lake, or into the jungle knowing that if it gets wet your losses are capped at $25, and if you fall down, it won't be damaged. With the e-book, a fall or weather could result in expensive damage.
Travel -- the paperback book doesn't need to be charged, doesn't need a power plug converter, works during takeoff and landing, and doesn't immediately mark you as a person who is worth robbing.
Portability -- you can carry a lot more e-books than books.
Search -- e-books are great for reference texts, where you intend to do a lot of searching for content, rather than linear reading. paper indexes are good, but not as powerful.
Markup -- e-books have a 'highlight' capability, and some note capabilities that seem like they'd work nicely in an academic context, but I generally prefer my system of post-it notes, scribbles in the margin, and underlines, which doesn't cleanly translate to e-books.
Disaster Recovery -- e-books are replaced from backups, or not at all. books are replaced by homeowners insurance payout.
Public Image -- my iPad and Kindle both mark me as an early adopter tech guy. a book likely says nothing about me, unless the person looking has an opinion about the title in question.
Permanence -- I'm relatively confident that my paper library will endure for as long as there's somebody who values the content of the books. I'm not confident my e-book library will be usable in 10 years. (between DRM, format incompatibility, and accidental data loss.)
Images -- books currently beat e-books pretty solidly when it comes to imagery. I can't really imagine getting a Photography book as an e-book at this point in time.
Visual ease -- e-books let you adjust the font, but paper books are often (though not always) easier to read.
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Or maybe e-books are superior in every single way, and I only buy books to fetishize the smell of wood pulp, whilst filling shelves to signal to my wife, kids, pets and good friends that I'm intelligent.
sigh