Okay just a hypothetical situation here.
Imagine that the INS somehow managed to get all of the illegal immigrants out of the USA tommorow. Everyone goes 'YIPPEE!!! No more evil mexican immigrants ruining our country!" But then we realize that there are no janitors, farm workers, construction workers, or any other essential but low paid people in the US. (Alright this is a little bit exaggerated but most of these positions are held by aliens) BUt we have to ask, why are mexican immigrants the only one taking these jobs? Because that job is below me goddamnit!! I Wont do it because I can find something better!
Personally I think we should be letting far more of them in legally than we currently do. America is supposed to be the land of opportunity, a nation of immigrants. But we are so afraid of mexicans 'stealing our jobs' that we can forget that.