All of the financial forces associates with illegal immigration -- particularly the latent demand for cheap and unregulated labor (I believe the latter is more important to employers, based on the fact that illegal aliens can actually command pretty high cash wages where I live) will still be in place even if 10-20 million illegal immigrants are "legalized" in this country. Once employers are forced to incur things like payroll taxes for these people, the whole process is going to start all over again with another generation of illegal immigrants.
That kind of blows a hole in the "low wage workers" argument doesn't it.