Here's a good post on Google Plus and what they called "Social Networking Fatigue" from GigaOM.
While Circles is pretty much the crown jewel of Plus and its ease of use allow for users to control who sees what they post and share, lack of use as time goes on actually exacerbate the privacy question.
So far, I don't seem to have that issue. And just because this situation of users simply putting new contacts into a default circle does not mean they cannot go back and made changes.
Rather, the re-sharing is a bigger concern for me than who to put into which circle. But I encourage you to click through and read more about it.