there's still lots of ways that buggy clients will simply blast to public relays. so it's definitely not perfect. But theoretically it could be better in many ways if done right
Discussion
Yep, my current issue is with DM relays that auth. If it is a personal DM relay that the user wants to filter who can send them DMs (only follows for instance), I think auth is cool. But if it is a public/common relay, then it's not cool.
I don't know exactly how to determine when to auth or not.