Given our current system of social benefits, the poor operate at a negative balance when it comes to "taxes contributed" vs. "tax-funded dollars consumed". The more poor people you add, assuming the programs and their parameters stay fixed, the more money needs to be paid in taxes by those who operate at a positive balance (i.e. paying in more than they take out). At some point, the system just breaks down. Imho if the level of social benefits in this country were similar to what it was during the last great wave of immigration, the power brokers would be alot more inclined to throw open the gates. Thing is, there's no going back.

(This is where I insert my plug for the FairTax, pointing out how it would mitigate the motivation for people to immigrate illegally.)