This does not include stand-alone services. Now we want to disallow access for everybody else. This is an insecure configuration unless legitimate connections might come from many diverse networks. The answer is that any publicly accessible machine is necessarily insecure and vulnerable to security problems. Hence, we should take proper steps to minimize the vulnerability.
Physical security is the first layer of security.
Home users probably need not worry about this too much. However, in a public environment, this aspect of security is a much larger concern. If users can reboot the system, it is trivial to manipulate the data on the system. Whenever possible limit user access to the console. System Security is a topic all by itself and addresses issues such as restricting user accounts to the minimal necessary privileges.