I think I agree, Cpt. Nate.
While it’s not always a given that there will be hostile foreigners, it’s a possibility that is too important for a proper government to ignore. After all, national defense begins at the borders.
We’re way past the time when nations would politely declare war on one another, and the rules of war were followed by honorable gentlemen. In an age of suicide-bombers, it is clear that savagery still exists in the world, and a proper government should defend the nation’s citizens against its enemies.
Certainly America is past “give us your tired, your poor”. Some traditions need to go away, but for those who truly crave freedom and have the initiative to handle it responsibly, our borders should be opened.
Many immigrants to the United States come here because of a job opening. They work for an American company that recruited them out of a foreign university, and while they work on a temporary Visa, they become “integrated” into America culture. When they desire citizenship, they are tested, they’re background is checked, and eventually they become citizens, fully protected by our laws.
Political refugees are a bit different. I could see some sort of halfway-house program where refugees would stay for a while, look for work, and possibly find some naturalized “sponsor” for a temporary Visa. (Who pays for them? I’d say employers of low-wage-workers, since we’re talking about a hypothetical “pure” Capitalist government.) Those that don’t find work, can’t support themselves, or have any sponsors on the inside are turned away. “Don’t have to go home, but you can’t mooch here.”
Willy-nilly border-crossers? I’m in favor of a strong border patrol pushing these guys back over, but maybe a few of those halfway-houses could be just on this side of the line for them. If they’re not ready to move here and get busy, obey the law, and keep out of trouble … well, we’ll have to toss those pesky Canadians right back over. 
But legal citizens bringing in mail-order brides? Foreign-exchange students arrying for citizenship convenience? Naturalized immigrants sponsoring relatives? I don’t know … are these enough of a problem for the Feds to get involved? I mean, anyone can be a spy or terrorist …
Just as a side note, notice that Dr. Binswanger is making the case for open borders at NYU next month. Link here.
Cool! I hope a trasncript is published … I’d really like to hear that lecture.