True but once you've got that original library of books on your reading device on what app are you going to purchase additional books on?
On my ipad I have iBooks, Marvin and Kindle. The first app I open is the Kindle due to the 180+ books that already sit on Kindle's cloud. And I doubt that is going to change.
As an utter aside I do have an e-ink reader. I went from an ipad and kindle to an ipad mini and then back to the former. Reading on an e-ink device a fall asleep reading on an ipad mini the light kept me awake...