User blog comment:Najevi/Cleaning up the sidebar when "fail to locate" a WikiPage widget/@comment-13301-20091222195749

Najevi. One of the main reasons to tell a user to create an account is to get rid of the ads. If it were an opt-in, then new users could feel a little deceived. new users won't understand what option change in preferences to get rid of ads.

Also, the ads seems to me completely offtopic in this blog post