This "openness" of the Kobo is a smoke screen since you have to remove DRM from Amazon books. And if you do that, then you might just as well convert the file to another format. So in the end, all readers are created equal as far as openess is concerned. None is really able to read books from all stores directly, but they all can handle it if you know how to disinfect and convert the files.
Yes, epub readers give you the choice of more stores without DRM removal --- but for me Amazon is by far the most important store.