Border opening can either be harmful or be helpful to a country, it works vice versa. It can be harmful in the sense that bad goods and services can be brought in,weapons can be smuggled in, illegal migration of non citizens can have quick access to a country when the border officials are corrupt.
It can also be helpful due to the price of goods and services because there will be competition in the market place there by making goods cheaper, it also strengthen international relationship.
What happens to the culture of a country when you systematically import immigrants into a country, illegally or not, who don't conform to the values of said country?
Just recently -- Syrian refugee goes on mass stabbing attack targeting children and women:
https://news.sky.com/story/syrian-refugee-who-stabbed-british-child-and-five-others-in-annecy-knife-attack-was-denied-asylum-in-france-12899146Turns out many of these Syrian refugees don't adapt to westernized culture very well.
The crime increases, and more cultural fragmentation within society leads to weaker social capital and structure. Sure, if you want cheap labor just bring in cheap workers from other countries. But there are long term impacts that go beyond the economic benefits. Culture can be lost in less than a generation -- a country is not a country without closed borders.