Sure, but that would also be extremely easy to spot, ban and restore. And then you would have to be
very determined to access this special wiki, and not just spamming wikis in general. A problem with a wiki is that somebody can go in and redirect a link redirecting (to f.ex. a D&D site!!

), and it would be pretty hard to spot. I would have to go through the logs every day to search out stuff like that.
With both registration and a post count requirement, I think we'll practically weed out all spammers, making the wiki quite easy to maintain.
Of course, granting access on an individual basis would also work, but I'm not sure if that's easier than just having a post requirement.
SGL.