I have had the honor of exchanging several emails with Mike Grehan about some of the theories spreading around the forums and found in many articles at SEO sites. One such theory we discussed was the concept of "themed sites" and how you need a site within a specific theme to stay within that theme in order to rank above your competitors (keeping every other variable as equal). So people were taking the extreme action of buying a new domain name for each theme. Then only having information about that theme on that new domain.
Now Mike is about to release his 3rd edition of his search engine book and he has been researching some of these concepts in great detail. With his permission, I will try to convey what his thoughts are on this topic, but not take away the deepness of the "why" found in his book.
Mike asked Daniel Dulitz from Google this question on 'theming' (is that a new word?). ""Utility" and "depth" really should be measured by a site's users." What I understand this to mean, based on the examples given by Daniel, is that a site will not hurt in rankings if it contains pages off topic to the real essence of the site.
Example; I have a site on the Smurfs (why did I pick the smurfs, I have no idea) but on that site, I have a page or two on how I block spam with spamassassin, will that hurt my site for ranking well with Smurfs? Not at all, according to Daniel.
Now, will a site with detailed information on spamassassin rank better then the page found at the Smurfs site? Of course, at least that is the goal of the search engines.
So when people discuss themed sites and how they are the only way to rank well, it is believed that some are misunderstanding the concept of what a "theme" is. The search engines think of it as the "utility" and the "depth" of the site (in pages and content).
Disclaimer: I hope I did justice to what Mike has graciously communicated to me. Also, I do not have a page on smurfs, nor do I have a page on spamassasin.
I posted this over at Search Engine Watch as well.