Alex Tabarrok has a very nice and very short piece at the Atlantic, The Case for Getting Rid of Borders—Completely. (HT Marginal Revolution)
In the Soviet era, there were walls and guards with guns, and we deplored that people were not allowed to cross the border. Is it that different that the guards with guns are on the other side of the walls?
If you're a liberal, you should cheer the policy with the greatest chance of elevating the world's poor and reducing global inequality. If you're a conservative, believe in the rights of individuals and freedom, don't like minimum wages, unions, protectionism, and government control, it makes little sense to switch sides on this one issue.