I suspect that Google+ will launch an equivalent feature, I think there is some real incompatibilities with the way that Facebook is handling it and Google+'s current privacy model that they are pushing.

If you share something with a group-circle and then someone joins the group then they can see your post; you are suddenly sharing with someone that you didn't actively intend to. Right now if you share with circles and the people in those circles changes it tracks the changes, but since you are the only person who can change the circles that doesn't provide any similar issue.