mals69 wrote:Googles system surely smart enough to know not to keep spidering a private forum ? It knows not to show any private forums posted material in their search results
As Eelke is trying to explain, Google and other bots do not "know" anything. All they do is follow links.
If a usergroup is not given permissions on a forum, then there will be no link for follow. If you give the Bots usergroup permissions on a private forum, the link will show and Googlebot will follow and index that private forum. Those indexed links could probably not be followed by a guest, but the private pages would still be indexed and cached.
As for drathbun's question on motivation to do this... A searching guest might find an interesting subject, but not be able to see it except in cache. That guest might be motivated to register to participate in that subject, or at least read it in a non-cached page.