Perhaps most of us here at MR, at least in this thread, are not locked down to any store with any reader we own due to our ability to strip DRM from our Kindle and/or ePub books. Then using calibre we can easily convert one format to another.

The average reader owner probably would have trouble stripping DRM and converting, either because of lacking the tech skills or even just not knowing that these options exist. And if they have trouble stripping DRM and converting, I should think they would also be tech challenged when it comes to transferring ebooks from one reader brand to another, for example transfer all their books from a Sony Pocket Reader to a Nook STR. I've never seen any reader that came with an easy import/export function for transferring books to another manufacturer's reader! Have you? So in my opinion, the average reader is locked into whichever device they buy, though not necessarily locked into the store associated with that device. Therefore JSWolf's old standby argument doesn't support buying Sony over any other major brand, as all major reader brands are going to be a challenge for the average user to transfer books to a reader manufactured by another company.