FWIW, when Borders went out of business I ended up getting a Kobo (plus two more for the family) I sometimes buy from the Kobo store, but I also buy from Amazon and strip the DRM (note I do buy the books - not pirate). I also have bought from Google Books, since Kobo supports Adobe DRM I can load the Google Epubs to the Kobo just fine. I also have old purchased prc files from fictionwise (yep buying ebooks before it was cool
) originally read on the Palm III, that I converted to ePub. Oh and I have a few purchased straight from O'reilly. And some DRM free books from Baen books for my cheesy Sci-Fi fix. and free books from manybooks.net and Project Gutenberg. I manage my library using Calibre on the PC.
Point is, there is no reason not to buy books wherever (except iBooks). Even though you can "side-load" books with any of the majors (except apparently Google Books) you don't get all of the features of the service on the "side-loaded" books, like syncing position. It would be really cool if Google would let you upload epubs to your Google Books account and then do all that, but neither they or any of the other majors have tackled that challenge yet.