I make no excuses for people who come here and don't follow the rules, and I don't know about where you live, but most of the undocumented workers I assume you are referring to where I live, are doing work that nobody else seems that excited about doing. I doubt you will find any working for any major companies like UPS or Starbucks. They all require ID's and SSN's. They certainly cannot pass a SIDA background check.
I am interested to know what jobs these undocumented immigrants might be taking that American citizens would want? There is not a large variety of people standing at the day labor sites here. I would assume if there was a large demand for that kind of work, we would see more diversity in those who seek it.
Again, I am not making excuses for people breaking laws. I am asking who wants to do the work they are doing and what jobs are they taking from citizens?