This is basically what I was trying to say in my post below, though I wasn't addressing the question of skills:
Is it better to allow a doctor from Nigeria to come to the US and increase his standard of living from being relatively well-off in Nigeria to being relatively well-off in the US, instead of allowing a farm worker from Mexico come to the US and transform the situation of his children from being hungry, sick, and illiterate to getting basic nutrition, health care, and education? And is it okay to deliberately try to deprive the poorest parts of the world of their best-educated people?
This in turn relates to the still deeper question of why we allow immigration in the first place: is the goal to improve the US, or is it to improve the lives of the individuals who want to immigrate?
For me, both goals matter, at least to some degree. I'm more sympathetic toward that poor Mexican farm worker than I am toward the Nigerian doctor, but the Nigerian doctor will probably make me (and the rest of the US) a tiny bit richer than the Mexican would. To me, those two effects roughly balance out. As a result, I'm not particularly in favor of changing the rules to only allow high-skilled immigrants in to the US. I can see the logic of it (it's really an economist's logic, after all), but it doesn't satisfy my sense of morality.
And this is yet another good point from Kash, who's rapidly emerging my favorite voice in the immigration debate:
current population growth in the US is only about 1% per year, including immigration. That is near the lowest levels of population growth ever experienced in the US - only in the 1930s was it lower. I think the US can handle a bit more than that.
