I´m not sure about how to prove that you bought the book either, apart from some sort of confirmation form from your store.
It was just a spontaneous idea - I have preordered my books anyway, so if I need anything on my pc for any reason, I can always scan it. Still, I believe that there would be situations when this might come in useful, like game mastering a multiplayer scenario or something like that (when people waving around hardcover books do little more that endanger those guys standing around them).
But then, I can live without the pdf Or the book, as long as I have either.