O'Reilly Rough Cuts - Beta Books?

As reported by Linux Watch, O'Reilly publishers are starting a new trend, beta books. Dubbed "Rough Cuts", these are books that have not been fully edited or lack complete formatting. I think this is pretty interesting. You get to see the book during its production when you purchase a cheaper online access pass. You can even send suggestions or bug fixes to the publisher to get incorporated into the final product. So what's the point of buying a book that's not even finished? You get to see it way before it comes out as a book. Take this Ruby on Rails Rough Cut for example. It was released as a Rough Cut in January and won't be released as a print book until August. The online version is generally cheaper as well.