One quote, from AEI:
There is a stunting statistic that I almost always have to give these days since hearing it. If you look at all of the PhDs in the US in the STEM fields, 56 percent were foreign born. So we are able to attract very smart people from abroad, keep them here, and have them work.Yes, a handful of those brains may spy for their original homeland, more of them will return "home" at some time or the other, but many of the brains will spend their most productive years in the U.S., years in which they do good science, create innovations and innovative enterprises, and generally make the U.S. better, most importantly by making it a place where others want to come, to learn and maybe work.
Other things being equal, it's better to export ideas and things, and to import people.