1. Open borders are an extremely important component of the free market and human liberty. The labor market is roughly 70% of the economy. Labor is the main product that most people around the world have to sell. Immigration restrictions massively distort this market, and deprive literally billions of people of the freedom to sell their labor to willing employers. So even if open borders made all other policies much less pro-market and pro-liberty, the (open borders + side effects thereof) package would almost certainly constitute a net gain for free markets and liberty.
Click through to read the rest of his argument, and the discussion thread. Economics blogs tend to have more sane, less belligerent commenters than most sites.