I'm personally in favour of leaving it as-is. If nothing else, I rather like that the forum is "closed" as an exhibit for search engines and other "drive-by" traffic (i.e., what is posted to the forum basically stays in the forum). I also think that it encourages users to join a community of CDP users. As a "social enterprise," this community will be vital to sustaining CDP in the years to come. While I feel that people shouldn't be forced to participate, I hope would, too, that they would be subtly encouraged to apply a different ethic to CDP than they might with commercial software, especially as the core CDP software is now maintained and distributed without charge.
I suspect most messages posted in the forum will also have limited significance a few years down the line, especially as bugs are corrected and the system evolves. I suspect a FAQ, for example, might be a more valuable source of information for new users. If there is interest, perhaps a Wiki system of some kind might play this role. A one-stop shop for manuals/documentation, known bugs, idiosyncrasies, and other bits might be preferable to wading through the "noise" of old forum posts.