On one hand, it makes switching from something like Reddit easier. For people joining us in good faith, that’s a good thing. On the other, it’s open season for trolls and spammers.
That said, I don’t think an instance with less than a team of admins spanning multiple time zones should have open registrations. Admins have lives and need sleep, so a single-admin instance with open signups has a huge window of opportunity for bad faith actors to take advantage.
There are automod tools which can help with this, but I still think open signups should only be enabled when an instance can have an admin on hand 24/7.