I've noticed a number of attempts by editors to push links to their
websites into talk pages, where they are much less likely to be removed
or <nowiki>'d than links in articles. I understand the reason for not
having rel=nofollow in the article space; however, I can't see any
reason not to have rel=nofollow applied to links to sites outside
Wikipedia in talk pages, which would stop rewarding this behaviour.
The alternative of removing or <nowiki>ing these links, is both
time-consuming and counter-productive, as it prevents all links from
being followed, not just abusive ones, and putting all talk pages in
robots.txt would be even more counter-productive by preventing any
external indexing of talk pages.
-- Neil