Wiki spam is a form of link spam on wiki pages. The spammer uses the open editability of wiki systems to place links from the wiki site to the spam site. The subject of the spam site is often unrelated to the wiki page where the link is added. In early 2005, Wikipedia implemented a default "nofollow" value for the "rel" HTML attribute. Links with this attribute are ignored by Google's PageRank algorithm. Forum and Wiki admins can use these to discourage Wiki spam.
Wiki Spam is a wiki wide problem. It won't be solved but wiki wide.
Spammers add links to wikis, blogs, bulletin boards, and guestbooks so their Google page rank goes up. We need to all help keep an eye on this at all wikis and help clean it up where we can. ............
Well many seo experts agree that even nofollow links do actually pass on some link juice. And not to mention that you do need to have a mixture of dofollow and nofollow links for links diversification.