Carry the logic to its conclusion, that borders equal structure equal social good, then the greatest good would be borders around the states, cities and neighborhoods where one needs papers and permission to step outside their own property.
No, that’s where part of the problem is, boarders are not themselves a social good, as you could say charity is a social good. Boarders are just a tool that are morally neutral.
One could say that laws and rules are a good thing. Would you want to invest in a company that does not want to give financial details that follow a certain set of accounting rules? We all know what can happen when people can cook the books. It’s not that the having books to cook is the problem, it was the deception of saying we are following the rules look at our statements, but actually we are not.
At the same time, having those rules and laws does not mean that everything we do should be dictated by a statute. It is good though that we do have statutes to limit us. It is a good thing we have laws against stealing, rape and murder. It is also a good thing we have rules saying that if your going to put someone in prison for breaking those rules, we have rules that regulate that procedure. Again as good as that may be, that does not mean we have to extend rules to say that I must wake up at 6:45 AM. The rules and laws are neutral, it is our action of enacting and enforcing them that is good or bad, and that deals with justice and prudence.
We will always have boarders, be it de facto or de jure - either as they are applied in reality or regulated by law. If you don’t like de jure boarders wait til they are de facto. You think they harm the poor when they are de jure, when they are de facto, they are more easily apt to be abused by those with the most means to power. After a certain amount of people are in a certain amount of space, it will become far easier to have them enacted by law. You can either solve disputes with an arbiter or you can use resources to enforce the boarders yourself. At least by law you can be far more certain that your border today will be your boarder next year, unless you decide give it up for some reason such as to sell it for money.
If you make a boarder just to make a boarder, that would be stupid in my opinion, because it seems to me it lacks prudence. If you need to set up a boarder to regulate flow of something that sounds reasonable. Depending on the flow you try to regulate, that may be right or wrong, with regard to prudence or justice. That itself though is a separate argument, and I think the proper argument. Granted I don’t think I could justify having a boarder any more than I could justify having government to an anarchist. About the only argument you can make is that it is practical, which really doesn’t do a great job of justifying it.