Do you know which publisher sells for all your favourite authors? I certainly know for some of them, but not others. What about when you just hear about a book from someone? This is why people go to bookstores.
I'd be a lot happier if everyone went to no-DRM or at least interchangeable DRM so you could move to a different e-reader. Amazon and iBooks are the odd ones out. iBooks doesn't have a big marketshare, but Amazon definitely does.