There is a lot of talk these days about search engines devaluing purchased text links. To be sure, buying text links to manipulate one's Google PageRank or rankings in the SERPs is a widely abused practice that warrants attention from the search engines. The argument is that such attention is needed to ensure relevant search results dictated by natural, rather than artificial, linking.

As an Adsense publisher, I am often annoyed to find my web pages filled with public service announcements (PSA) and ads unrelated to the content of my site. Fewer relevant ads mean fewer click-throughs and ultimately lower Adsense revenues. However advanced Google's contextual ad serving algorithm may be, it is still a computer program (as opposed to a human being) trying to figure out what a web page is about.