Gavin recognizes that the NA is doing a difficult dance with their user community:
[S]ome people who have valuable knowledge would be put off from contributing if they had to give it away under GDL, and might prefer a non-exclusive licence which allows them to retain more rights. For example, the average Great War Forum member doesn’t tend to think in a Web 2.0 kind of way. But then they might be put off by the very idea of a wiki. Including as many people as possible has probably involved some difficult decisions for the NA.
This puts me in mind of a discussion over at Dan Lawyer's blog last year. Amateurs who are willing to collaborate on research feel a strong sense of ownership over the results of their labor. They don't want other people taking credit for it, they don't like other people making a profit from it, and they don't like seeing it misused*. Wikipedia proves that people will contribute to a public-domain project, but I suspect that family history, being so much more personal, is a bit different. Several of Dan's Mormon commenters feel uncomfortable entrusting anybody other than the LDS church with their genealogical conclusions. Of course, many non-Mormons feel uncomfortable providing genealogical data specifically to the LDS church. Getting these two sets of the public to collaborate must be challenging.
This same week, Rantings of a Civil War Historian published a fascinating article on "Copyright and Unpublished Manuscript Materials". The collectors, amateurs, and volunteers who buy letters and diaries on EBay feel a similar sense of ownership over their documents. The comments range over the legal issues involved in posting transcripts of old documents, as well as the problems that occur when people with no physical access to the sources propagate incorrect readings. Many of the commenters have done work with libraries that require them to agree to conditions before they use their materials, and others work in IP Law, so the discussion is very high quality.
* I think the most prominent fear of misuse is that shades of nuance will be lost, hard-fought conclusions will be over-simplified, and most especially that errors will propagate through the net. In my own dabbling with family history, I've seen a chunk of juvenile historical fiction widely quoted as fact. (No, James Brumfield did not eat the first raw oysters at Jamestown!) Less egregious but perhaps more relevant to manuscript transcription are the misspellings and misreadings committed by scribes without the context to determine whether "Edmunds" should be "Edmund". Dan discusses a technical solution to this in his fourth point: Allow the user to say “I think” or “Maybe” about their conclusions. That's something I should flag as a feature for FromThePage.