I think my primary motivation for putting down dead tree books is that the e-books I've bought and haven't read are invisible. They don't sit on my bookshelves openly shaming me for not reading them :-O
In Lem's Solaris they have paper books even on space station. (Love Tarkovsky's movie.)
But anyway, just to clarify, I mention that these apps exist because it could completely throw off the numbers as to how many "Kindles" (or "Nooks") there are in a given city. What if SF has tons of people reading Kindle books on their iPads? These devices can't be considered pure competitors to the Kindle (or the Nook), and their numbers are way, way too big to simply disgregard.
Maybe that's largely a personal opinion or maybe most people would agree with me, but either way my point is - yeah, it's not "too bad", and is perfectly fine, but it doesn't mean it's not significantly worse.