There's a very interesting NYTimes article about illegal immigrants contributing to Social Security and Medicare, as well as the taxes they do pay. "...the estimated seven million or so illegal immigrant workers in the United States are now providing the system with a subsidy of as much as $7 billion a year." Plus, these immigrants will never get the opportunity to access these benefits since they are not using their own Social Security Numbers.
Doesn't it always boil down to money?
Here's another example of how it boils down to money. Let's say you are building a house and the general contractor can't verify if the people building your house are all legal citizens. (We both know they aren't and he's paying them less than he would if they were.) If you demanded that everyone building your house be legal citizens what do you think that would do to the cost? Would it go up by 10, 20, 30%? How much more are you willing to pay and forego the granite countertops and dream kitchen experience?