Responding to a question about whether Google is/will implement "theming" (factoring in outbound thinks to sites that are 'on theme' with the search string), GG says that he is not sure he is a huge fan of of some theming arguments. He also says that it would natural that Google would want their scoring to reflect things that make a site useful for users. Link to quote
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I'm not positive that I'm a huge fan of the theming arguments that people have made--some of the most useful links I've seen are from "off-topic" sites--but I would definitely agree that it helps users to link to useful, relevant, related sites. So I could see where someday we might our scoring to reflect that in some part. If you see tons of links flowing into a site and not a single link to the rest of the web, then as a user I might scratch my head a little bit.
That's just my personal take on things though. I guess my short answer would be that it's natural that we would want our scoring to reflect the real-world things that make a site useful. In an ideal world, webmasters would only worry about making great sites for users, and Google would follow that to find the best sites that users loved, and score those useful sites highly.