Thinking about the matter more, I guess that being able to read Kindle books on other devices is of less importance, for the simple reason that you can't (currently) buy a Kindle book unless you own a Kindle (I bet igorsk will tell me I'm wrong about that, too!
). I suppose you might want to be buy a book on your Kindle and be able to encode it for Mobi on your smartphone, but being able to buy a book from Mobi and read it on your Kindle would seem to be the more useful capability.