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.
This is a really difficult subject since open borders may lead to a larger interchange of thoughts and customs, as well as higher economic activity, but they can also pose security threats, as you indicated. So I believe it's critical to create a balance between security and transparency. One possible solution is to implement an organization of checks and balances at the border. For example, heightened security measures such as improved screening and background checks for persons entering the nation may be implemented, yet those who are approved to enter may continue to travel freely. This would provide a balance between safety and the enjoyment of mobility.