"If you do a marketing site and you don't have an RSS feed, you should be fired."
Robert Scoble evangelist and blogger, Microsoft
According to wording in Google’s patent application (U.S. Patent Application # 20050071741) ‘documents for which there is an increase in the rate of change might be scored higher than those documents for which there is a steady rate of change.’ In other words, even well-optimized content needs to be periodically refreshed in order to maintain top search engine rankings.