As an administrator, the only time you would want to turn on the public stream is if you are a public hub and accept new signups. It makes it easier for administrators and moderators to moderate the public content on their own server since they can see all public posts in one place. If someone is posting illegal content or spam, a moderator can see it, and remove it (and perhaps the user too).

Even then, it wouldn't be a federated public stream that's in plain sight for any visitor. At most, it'd be a local pubstream in plain sight for anyone. Or a federated public stream only visible to local users.

At least by German law, hubmins can be held liable for what's happening on the pubstream because it's happening on their "website", and so they're responsible for it. And remember that most public Hubzilla hubs and the two biggest ones are German.