I'm going to lay out some facts that come together in a conclusion. See how many you need to read (in order) to guess the conclusion!
1)From what I understand, your average immigrant is significantly younger than your average American.
2) The United States has largely been in debt for the vast majority of its history.
3) Social Security, which actually currently runs a surplus, is going to start running unimaginably crushing deficits in the next decade without some serious reform and some serious luck
4) A major reason the United States has survived being in debt for over 200 years of its history is because it keeps growing in line with the long term growth of its debt, keeping the debt level as a percentage of GDP down.
5) England successfully reduced its debt level from bankrupt to solid-to-strong over the course of a century. Much of this was through fiscal discipline, but a major portion was growth, as well.
6) When people retire, they stop being productive.
7) The baby boom generation is hitting retirement age.
8) In the 1920s, the United States was the low-cost goods and resources producer to the rest of the world.
9) The Great Depression hit the US so hard because worldwide demand for its goods plummeted. The US largely shifted to the high-education, high-intellectual capital, branded, concept jobs that dominate it today.
8) China has a lot more people than us and is much more nimble because of the authoritarian nature of their government, so a lot of the high-tech, high-education ideas that have driven our growth since the 40s are going to them, and they have more of the natural-resource, commodity, manual labor, factory, etc. jobs because labor supply is still so high.
The United States, in order to have any chance of surviving as a world empire, needs to be able to outgrow its debt. Growth is going to become significantly harder with an aging population and a China that can, all of a sudden, start taking away a lot of the jobs that we have had an advantage in for so long. In order to deal with that, we need young people and we need to a) defend our dominance in the high-education jobs department, and b) replace our losses there with manufacturing and production in other areas.
Immigration provides a lot of that - immigrants are younger, and often productive in very different ways to people who grew up in America. Educated immigrants are especially valuable to the US. Restricting immigration is one of the fastest ways to turn us into Western Europe, but bigger, which isn't good, because the Western European model isn't so scalable, especially given our much, much larger military expenditures.
I've railed a lot against liberals recently, which I think is fair, given that liberals control both the White House and Congress, and the White House and Congress have had some TERRIBLE policy recently (stem cell research excluded - I love Obama's stem cell research policy). However, here, I get to rail against conservatives. The conservative notion that you can protect American jobs by restricting immigration, and that immigration being illegal is somehow indicative of immigration being destructive, is downright stupid.
Restricting immigration is just as protectionist as tariffs, which are also absolutely idiotic in 99.9999% of cases.
The reason immigration is monitored is to regulate the kind of people who can enter - people who can be productive and exist in our society without hurting other people. That should imply a qualification, not a quota... analogously, restricting who can get a driver's license may reduce the number of accidents on the road, but that doesn't mean it makes people as a whole better off.