Why Be Topical?
There are numerous ways the engines can run topical analysis to determine whether two pages (or sites) cover similar subject matter. Years ago, Google Labs featured an automatic classification tool that could predict, based on a URL, the category and sub-category for virtually any type of content (from medical to real estate, marketing, sports and dozens more). It’s possible that engines may use these automated topical-classification systems to identify “neighbourhoods” around particular topics and count links more or less based on the behaviour they see as accretive to their quality of ranking results.
I personally don’t worry too much about topical relevance – if you can get a link from a topic agnostic site (like NYTimes.com) or a very specific blog on a completely unrelated subject (maybe because they happen to like something you published), I’m bullish that these “non-topic-specific” endorsements are likely to still pass positive value. I think it’s somewhat more likely that the engines might evaluate potential spam or manipulative links based on these analyses. A site that’s never previously linked-to pharmaceutical, gambling or adult topic regions may appear as an outlier on the link graph in potential spam scenarios.
