Manual:Nofollow

There are many costs and benefits of using nofollow. Nofollow is is a value that can be assigned to the rel attribute of an HTML a element to instruct some search engines that a hyperlink should not influence the link target's ranking in the search engine's index. The configuration setting Manual:$wgNoFollowLinks is used to control whether a wiki's external links should have nofollow or not.

Benefits
The main benefit cited for using nofollow is that it deters spam. Mainly this benefit applies to pages where spam could be overlooked and therefore not be reverted. Nofollow mostly deters spammers who are sophisticated enough to know about nofollow about not sophisticated enough to realize they can still get a return on their investment even with nofollow in place. Nofollow unfortunately does not deter spammers if their motivation is to attract visitors to product pages rather than to boost pagerank.

Costs
Using nofollow will prevent a wiki's links to other sites from boosting those sites' search engine pagerank, which is bad if the wiki owner wanted to boost their pagerank. Turning off nofollow is especially a good idea if you run a private wiki (i.e. one that does not allow editing by anonymous users, and that allows registration by invitation only). In such a case, spamming is not a concern because the spammers lack access to edit. Some public wikis also avoid using nofollow because they want to encourage people to make useful contributions that add external links to their favorite sites. Another cost of nofollow is that there is not yet much capability for using it selectively; it has been argued, "Can you imagine a web that works as good as today's if every link was tagged with nofollow?"