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?
Best Answer
In composing and editing the question, I might have come up with the answer myself:
So "Public" provides availability while "Extended circles" provides distribution. These are separate traits.
Frankly though, why would I write something that semi-strangers may read, but total strangers may not? That is what the "Extended circles" does, and it doesn't make much sense to me. So in essence, I would always use both in combination.