Societies that take in "brain drained" scientists and others benefit enormously. Innovative and entrepreneurial French Huguenots contributed mightily to the launch of the Industrial Revolution in Britain. American universities benefited mightily from refugee German Jews fleeing Adolf Hitler. Today's Silicon Valley would not be what it is without its brilliant Chinese and Indian entrepreneurs.
The Swiss Federal Institute of Technology in Zurich (ETH) has the highest number of Nobel Prize winners of any institution in the world, no doubt partly due to the fact that 35% of its faculty is foreign. From Cordoba 1000 years ago to California today, the most intellectually stimulating places are crossroads for bright people from different cultures.
Societies that fail to attract foreign brains stagnate. Take Japan. Japan's homogeneity helped create the economic nationalism that drove the country for several decades, but today most of Japan's universities, research institutes and laboratories, think-tanks and elite publications, suffer from sclerotic inbreeding. Japan's current lethargy is due, in part, to the in-bred languor of Japanese intellectual life.
But aren't countries that export their "brains" impoverished by the process? It depends. Spain, for example, saw its best minds drained away for five centuries, notably following Fascism's victory in the Spanish Civil War. When Franco died in 1975, Spain's future path was not obvious, as evidenced by the attempted coup of February 1981.