The reason for a project like this is to provide a platform for content that is best published in a final form. Examples of these are books from Wikisource that have been transcribed, proofread ... a finished product. The same is true for collections of Wikipedia articles who currently are published, often curated, and sold on ebay.
In my thinking it takes away a function that is not really get enough thought and momentum from Wikibooks and Wikisource. This is the publishing side of thing. The relevant difference with Wikipedia that a Wikipedia article will never be absolutely one hundred percent in a final form. As relevant it the realisation that casual visitors are not necessarily interested in anything but finished products and consequently finished content does not get the public it needs.