One of the fallacies that I've seen cited at Daily Kos occasionally is that undocumented workers mostly do agricultural work that Americans citizens refuse to do.
Whether or not American citizens would really refuse to do agricultural work if it were well-paid and featured the same kinds of legislative protections as other kinds of work is an open question.
But as my local newspaper points out, undocumented immigrants from Mexico are a central part of the local economy, and a fixture in one of our healthiest sectors (construction). When these men don't show up for work, fearing a Border Patrol raid, houses don't get built.
The story can be found here:
http://www.lcsun-news.com/...
If you read it, you will see that local Anglo contractors practice a kind of "hear no evil, see no evil," when it comes to hiring undocumented workers. They all do it--or they hire subcontractors that do it--and they are brazen enough to admit doing it to the press. You see how some of them "solved" their illegal worker program temporarily--by firing their guys for a couple of weeks so that the workers will not be deported. So here are workers without any job security, either.
Our current immigration policy, or lack thereof, harms everybody: American citizens and undocumented workers alike.