I agree with that and think they need to implement an obvious extension: shared circles.
I find I share to "All your circles" a lot more than I share to "Public", because I like knowing who's reading my stuff.
Or: I have a circle containing all great hackers. I then post the link to it here on Hacker News so everybody can have that whole circle with a single click.
At the same time, I'd still like to be able to mutually join groups as well, and it's easy to imagine scenarios where I would add a group to a circle (or where they'd be coincident). The two concepts are complimentary.