I'm guessing the use-case here is "I have a particularly nerdy observation I'd like to post; I want my nerdy friends to see it, and I'm happy to have random nerds (from Reddit, HN, etc.) wander by and comment on it, but I don't want to bother my family/colleagues/non-nerdy friends with it". Does sharing a post with a circle and "Public" accomplish that, or is it identical with making a purely Public post?