Here is an economic reason for this being a problem:
When an illegal immigrant comes here and "works two jobs", they are both breaking the law (working under the table or using false identities for tax purposes) and they are depressing the natural labor rates built into a capitalistic economy. Consider that the illegal is willing to take almost any job for significantly less than their legal counterparts (being legal immigrants or citizens). Because employers can now hire the same number of bodies for less (whether knowingly or not - that's a different topic, really), the rate which the employers are willing to pay is lowered.
The illegals also are willing to take jobs without health benefits. After all, there have been numerous studies that show that illegals avail themselves of "free" emergency care when necessary - despite the fact that they have no history of paying into the system that they are using (through insurance or medicare/medicaid). So, they get their health care (which is a substantially different thing from "health benefits") paid for by you and I - and anyone else who pays their taxes, health insurance premiums, and doctors' bills.
wow, i had no clue about the health care benefit. I also didn’t take into account about the two jobs thing. One job can turn around a life and it might be ruining an actual citizens. The health care can also put a counter strain on the family and maybe have the hard worker suffer more.