The south park episode was funny but in reality they are taking jobs. People were making a higher wage at skilled trade jobs. Companies hire illegals at a much lower rate. The people have a life based on their cost of living and a higher wage. They can't afford to take the lower paying job. So there are forced to go on employment or other social services (welfare). The illegal is working under the table but still able to get free housing, welfare, etc. That illegal is a huge tax burden and has made the skilled trademan a tax burden. You can probably search PBS for the show. There are also stats all over the internet and just a little common sense.
The people who aren't against deporting illegals or not allowing them to have services paid by us are the ones who benefit from them (own company hiring them or there corporation is affected by an industry will make larger profits because of illegals).