Speaking as a writer, I wouldn’t do something like this; I don’t generally like writing in other people’s worlds in any event (and when I do, I go public domain — see Fuzzy Nation) and I don’t like the terms that are on offer here. And of course I have my own things to write. Likewise, I would caution anyone looking at this to be aware that overall this is not anywhere close to what I would call a good deal. Finally, on a philosophical level, I suspect this is yet another attempt in a series of long-term attempts to fundamentally change the landscape for purchasing and controlling the work of writers in such a manner that ultimately limits how writers are compensated for their work, which ultimately is not to the benefit of the writer. This will have far-reaching consequences that none of us really understand yet.
Scalzi on the Kindle Worlds program.