What I see is that even in today's MOOC/highly-connected world, I'm missing out on interacting/working with the brightest minds in the world, who all seem to be currently concentrated in the US. I can only go so far by watching their talks online, reading their research papers or studying their code. I wish to live/work in that environment for at least a few years.
Not everyone can get everything they want, I get that. But then, I see folks who are neither the best nor the brightest nor the most hard working getting to the US by gaming the system and that hurts.
PS: This is from the point-of-view of a foreigner from a less privileged background. There is a whole other gamut of concerns of US citizens about their own jobs, assimilation, social concerns and all that and I am aware of that.