I would assume that this is standard, especially for smaller companies. Offer people the end product for half the price if they proofread and help with finding errors before release for example. Not sure to which extent this is done by Mongoose though, but looking at how fast errors are found and reported here, I would say that there is room for improvement.I still think a team of volunteers before anything is even released on pdf would go a long way
Fortunately we are talking about the PDF-version, which can be updated more easily. The only problem is to keep track of everyone who is entitled to receive an updated version once they bought it.