In: Computer Science
1- Is there such a thing as an ethical reason for unauthorized access to a computer system?
1) the key word here is "unauthorized". Accessing an Unauthorized system by law, can be ethical, i think, although not legal. I think this is the point where you use your judgment of right and wrong. If it meant getting into some criminals personal file to catch him then i would feel its right.
2) Yes, you're stopping someone exploiting vunerabilities in services you expose through your router to the Internet, by not having any.
But, there are other possible attacks. What if you initiate a connection from inside, and the attacker has compromised the other end? Or if the attacker is pretending to be the other end (man in the middle). Or what if the attacker exploits a vulnerability in your router software? Or breaks into your house? Or social engineers you into telling them your mail password? Or steals your offsite backups?
So yes, it works as described, and if you do not need to expose any services to outside you should block everything incoming: but there are other ways to attack your network.
Hope this helps you. Please give an upvote. Thank you.