The big advantage of a switch to ePub, and ePub exclusively, is that it allows more advanced formatting to be used. If you look at the recent Lord of the Rings, for example, the formatting of the ePub is almost entirely consistent with the LIT version (
A Comparison of format limitations based on The Hobbit ). So a world-wide release like this one, even of an ebook where investing in better layout might pay off in higher sales, is still constrained by older formats. On the plus side, it does seem that LIT is the minimum standard now, and MOBI and others are downconverted from that.
My problem with ePub-only is that it is effectively Adobe ePub-only and Adobe Digital Editions isn't a very good ePub viewer. It isn't completely terrible if you accept its concept of operation, which is that the publisher is always right and the reader should have virtually no control over layout, but even then I would argue that it should (optionally) implement hyphenation for example. If you are used to a Reader that gives you some control over the layout of text on the screen then Adobe Digital Editions is not for you.
So if the UK publishers stick to their ePub-only and DRM-mostly strategy, then FictionWise's plan to add ePub to eReader Readers (including eReader-like DRM) starts to make very good sense. Anyone who wants to stick around in the ebook market has to have an ePub strategy, and it probably has to include a unique DRM scheme for their particular reader.