The point isn't to limit the viewership, but to target the people you annoy with your notifications.
Sharing to a specific circle only implements privacy control, but it's much too heavy handed - nobody else can see it. Also, you can't change sharing options after the fact, so choosing the right options becomes particularly important.
I think the problem is more that just because random person X "has me in their circles", I've no idea whether they're interested in my posts about cheese or my posts about Latvian clog dancing or my posts about C++, so I can't assign them to my topic-specific circles for targetted notifications.
Sharing to a specific circle only implements privacy control, but it's much too heavy handed - nobody else can see it. Also, you can't change sharing options after the fact, so choosing the right options becomes particularly important.