That means that people you know aren't interested in a particular topic don't have to see something that you post. Of course, on Twitter you can restrict your followers but that rather goes against the nature of the service. And if people want to follow you it is reasonable to assume that they are interested in what you have to say.
But given that many people now use social media, Google plus and its Circles seem a real innovation.
Will I one day abandon Twitter? Probably not, but Google plus is a welcome addition to online communication.