Hi all, bug 42594 https://bugzilla.wikimedia.org/show_bug.cgi?id=42594proposes changing the default value of $wgNoFollowLinks https://www.mediawiki.org/wiki/Manual:$wgNoFollowLinksfrom true to false. The status quo is that, by default, external URL links in wiki text will be given the rel="nofollow" attribute as a hint to search engines that they should not be followed for ranking purposes as they are user-supplied and thus subject to spamming. If the change is implemented, you will need to change your LocalSettings.php to switch $wgNoFollowLinks to true if you want to keep the status quo on your wiki.
The argument for the status quo is that nofollow deters spammers. The argument for the proposed change it is that it's better for the Internet as a whole, and arguably for the individual wikis, to have the links followed for ranking purposes. I'll focus on the arguments in favor of the change and let others rebut them.
Suppose you run a wiki, wiki.foowidget.com, devoted to documenting your software application, FooWidget. If you link to, say, the main foowidget.comsite or to a vendor that stocks your software, would you not want to improve their pagerank, since this benefits you?
The same goes for, e.g., nonprofits that are promoting a cause. If you run CancerWiki and there are a bunch of links on your site to the American Cancer Society and other allied causes, would you not want to increase their pagerank? I think that in the wikisphere, what we commonly see is wikis devoted to niche interests they are trying to promote or share information about. The reason they link to certain websites is that a community consensus has decided that those sites are useful for effectively promoting, or informing people about, those topics.
If the links are spammy, then the editing community at that wiki should revert those spam edits. If they do so promptly, then if they have any effect on pagerank at all, it won't be for long. A well-maintained wiki will mostly have links to good sites, and the effect of the pagerank boost those provide will drown out the pagerank boost that goes to the short-lived spam links.
Also, we have other antispam tools that are way more effective than nofollow at deterring spam. Sites that mirror a wiki may not apply nofollow anyway, in which case those links might still increase the spammers' pagerank, regardless of your nofollow setting. It's hard to reduce the benefits that accrue to the spammers, except by vigilantly reverting their edits; it's easier to increase the costs that the spammers incur, by using CAPTCHAs and the like.
$wgNoFollowLinks was introduced in MediaWiki 1.4.0 as a setting that defaults to true, so I'm not sure that we really gave the other option much of a chance. Also, well-designed search engines should have other measures too for sorting out what's spammy. There should be some sort of algorithm for identifying wikis that have been overrun by spam, much as the search engines have ways of figuring out which sites have a bunch of links just for SEO purposes.