People aren't simply interchangeable units of economic output, and when they live together for long periods of time they develop unique and distinct cultures that are dear to them. Letting the entirety of the world into your country without stops is cultural suicide. If you put any a value in preserving culture you will find it ethical to put some limit on immigration. Assimilation is possible but there is a limit to the rate and amount of foreigners a place can absorb before changing significantly.