I could just as easily talk about the "obvious increase in utility afforded by real physical books."
This isn't even a case of rational or irrational behavior. It's a case of an economist misunderstanding the real value of a product to consumers.
Just because people buy ebooks doesn't mean they view them as having greater utility than real books. Does a movie from iTunes have greater utility than a DVD? An MP3 versus a vinyl record with beautiful sleeve art, gatefolds, and liner notes? Is an iPad "magazine" more valuable than a physical one?
Digital simulacrum of real-world objects are popular because they can be purchased on a whim, and produce instant gratification. That doesn't make them inherently superior.
One author friend of mine, Margaret Weis, has seen ebook sales outpace her physical book sales this year. At the same time, her physical book sales have not suffered at all. What has happened is that people have been buying ebooks for instant gratification, then going back and purchasing the real thing later. That would seem to speak directly against your opinion that ebooks have an "obvious increase in utility."