"The more doctrinaire libertarians see the benefits of free international trade in goods, and extend the same reasoning to free international movement of people. But goods do not bring a culture with them. Nor do they give birth to other goods to perpetuate that culture."Bingo.
If I had to describe where I part company with (some) libertarians, it is on exactly this point - that I think they frequently tend to underestimate the importance of culture. This often makes them both unwilling to defend western culture in a meaningful way, and enthusiastic about widespread immigration, even if it is comprised of individuals unlikely to share (or even deeply antithetical to) the values of western society.