The problem with the public communities is that it doesn't take long before they're dominated by those who have the most free time on their hands. Free time to post, free time to respond, free time to upvote/downvote and direct the discussion. Generally speaking, the people who have the most free time to be online all day, every day are not the same people who are out there getting things done, doing interesting work, and otherwise having a good time in the real world. That's fine if you're looking to waste time clicking around on Reddit or Twitter, but it's not often conducive to forming real friendships, relationships, or communities. Obviously there are exceptions, but the more public and noisy the community, the more time you have to invest to filter through to the weak signal in all of that noise.