Obviously I can post something to the "Public" circle and that will be visible in the stream of those who have me in any of their circles, and visible to strangers viewing my profile.
Also, I can post something to my "Extended Circles" and that will be visible in the stream of those who have me in any of their circles, and in the "Incoming" stream of their friends, but not visible to strangers viewing my profile unless they are in "Extended circles".
Under what conditions would it make sense to post to "Public" and "Extended Circles"? What does this achieve?