The US does have a high rate of immigration, but on the other hand it's underpopulated compared to other developed countries. The biggest reason to loosen restrictions on immigration IMO is that without doing so the US is going to have difficulty with the payouts for social security and so forth as more baby boomers retire and there are fewer new entrants to the workforce. This is a medium-term problem (~20 years), left undone then it will be much more expensive to handle by the time it becomes acute, as countries in central and south America are more like to exhibit a labor shortage than surplus and will be competing with the US for the same labor pool.

http://www.ssab.gov/documents/IMMIG_Issue_Brief_Final_Versio...