What happens if...
 

I take a registered user and assign them to the "Unregistered / Not Logged In" group as their primary group?

Does it mean that, even if they log in, they will only see guest view?

Try it with a test account and find out.

It all depends on what you have that usergroup permissioned for. By default yes, that account will have guest only perms.

UPDATE: I just tested this myself and yes, the user given this usergroup can log in just fine, but can't do anything else the usergroup doesn't have permission for.

Thanks, Max!

I don't know if this will be a useful thing to be able to do—but it seems like it might potentially be.

This is the most interesting question I have ever seen asked on here lol ;p

I try!

Could be used as a banning option that might also confuse them enough to not turn right around and make a sock account after a real ban.