I think the real problem is that there's no easy mental model for the way information flows through the system. That's what metaphors are for. A "Circle" is not a metaphor. It's just a brand.
I don't think Circles intends to aid in setting social context for other people. It's intended to help the sharer set context for who will see their content. Google is betting that people will share more information if they can limit it to the right groups of people.
As a commenter on this content, I don't get to reset the context or modify it. I can either trust the initial poster's choice or I can dig in further. The mental model is pretty simple: I trust the poster or I take a look around the room and see who else is there. I think when people do dig in further, they will either see a small group of people that they recognize or a large group of people which will set them in "public" context mode for commenting.
So what if they use + the same way. Ignore them, move them to a quieter circle. And for their benefit, it defaults to a more protective privacy setting than Facebook, especially for pictures.