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I believe that the balance of the link outweighs the threat of penalty from low quality sites scraping your content - providing your not already seeing warnings in your Google Webmaster Tools. If your content is being scrapped there will likely be other links in the content anyway (as it's normally good practice to interlink your content within your own content). Removing the link added by the plugin wouldn't remove the links in the content and you would still face the same predicament as if you were to include the footer link. In the case of your content not being interlinked then the decision would need to be based solely on the quality of the domains scrapping the content and you'd need to do a little bit of research to find the sites that were commonly scraping your content and decide based upon the reputation of those sites. I don't think an answer to this one will be all that black and white as it's highly situation dependent. If it were doing your rankings major harm then you'd be given a warning in your Google Webmaster Tools and you'd know to disable the footer link in the RSS feed. But if you're getting no warnings, initial impressions of the scraping sites aren't too bad and you've seen no other issues relate to the links then I'd keep the link and hopefully benefit from the free backlink with my chosen anchor text.