Well, I suppose it makes sense from a perspective of keeping all your books on their server and only (re)downloading them as you want to (re)read them, but I don't work that way, and I don't really think most folks do -- I tend to keep my own backups when I'm not actually reading a particular file. It's nice to have the extra backup on their server, I suppose, but I don't really expect to be relying on it.
