The concept is pretty good - the reality is not so good. What it tells you is that many, many users and webmasters either (a) turned on and are now ignoring a site, or (b) perhaps don't grasp the concept of downloading or adding themes to their sites.

Perhaps it would be a more useful metric of "active themes on sites that have updated in the last 180 days" or "active themes on sites with more than 10 posts, as an example, to eliminate all of the "hello world" sites out there.