Everyone illegal is either doing a job that doesn't really need to be done or they are stealing a job from an American.
Because Americans would do these jobs if they were compensated for them. Thus, if an employer must have a task completed, then he has to compete in the labor market just like you must compete for your wage... Until, that is you start figuring illegal labor into the equasion.
Then it becomes an employer getting his work done for pennies on the dollar, while you have to compete even harder for your wage.