To tell you the truth, I believe if you do it the right way they can be a real strength to a country's economy. We so many people move around in the world. I am not saying that we should exploit them if that is what your thinking I am saying that they can replace some of the people that leave from a country.
The United States is a country where in my opinion some, if not most of the economic growth comes from immigrants that left their own countries in search of something they were lacking in their own country be that wealth, education or something else. I am not talking about illegals (Which in my opinion add to the economic growth as well), I am talking about the immigrants in search of an education or research grants or the ones seeking new markets for their businesses.