E-books are useful for teaching when an unlimited, or at least very high, number of users can access the same book at the same time--this is superior to putting a single (or even multiple) copies on reserve for students. There are also times when I need to look up something in a book outside of library hours, and access then is priceless.

BUT

I recently had the rude experience of discovering that the Ebsco e-book I had told my students they could access on line was limited to being checked out for 5 days by ONE user at a time! That is so ridiculously inferior to a hard copy I don't know where to begin; obviously Ebsco keeps on charging the library a yearly fee for this miniscule access, so year-over-year, it ends up being more expensive than getting a hard copy.

Also, Duke can't lend ebooks via ILL. The more titles we acquire as ebooks, the less useful we shall be to our ILL partners. Not a good idea, when (e.g.) UNC-CH has many out-of-print titles in my own discipline (classics) and other disciplines that our collection does not.

And lastly, what happens when the company who controls the database when the ebook originates goes bankrupt or changes hands? That information will no longer be accessible, whereas a hard copy will be on the shelves as long as human ingenuity can preserve it. We are not beholden to any outside commercial entity for hard-copy titles we have already acquired, but e-books make us servants to the e-masters of those books.