At Search Engine Strategies, we discussed the problem of aggregators and spammers taking blog content and using it to build aggregation pages (with AdSense links, naturally) that compete with the source blog for traffic. In some cases, these aggregation pages present tightly focused content about high-value keywords.
In a perfect world of search, the search engines would be able to determine that these aggregation sites are spammers and block them accordingly. However, because the aggregation sites use legitimate content, the search engines can’t recognize them for what they are.
See on blog.ericgoldman.org