Competition In the Free Textbook Market
Ben makes an excellent point in saying that "the NC license is incompatible with strong copyleft licenses such as the GFDL used by Wikipedia," because this is true. And the Wikipedia's GFDL is incompatible with the CC By-SA license used by Wikieducator. And Wikieducator's CC By-SA license is incompatible with the CC By-NC-SA used by MIT OpenCourseWare. And MIT OCW's CC By-NC-SA is incompatible with GFDL used by Wikiversity. And Wikiversity's GFDL is incompatible with the CC By-SA licensed images on Flickr. The higher-level point is that "copyleft" clauses (which require that derivatives be licensed with ~exactly~ the same license) are the biggest legal problem with open textbooks and open educational resources generally. Every copylefted open educational resource is incompatible with every other copylefted open educational resource with a different license.