Hit the nail right on the head. Pay low wages, provide no health care or benefits, no hassle over getting rid of workers they do not like, etc.... Additionlly, these illegals send about 17 billion back to Mexico, it does not stimulate our own economy.
BTW, I agree with you - illegals do depress wages, etc., but the question is how to deal with the problem. A lot of people seem to think that if they just get angry enough, all of the illegal immigrants (who are not only Mexicans - there are also many, many illegal Chinese and other immigrants) will simply leave.
I would like to see a structured program that would deal with the group that is already here (regardless of their country of origin), but at the same time, since the great majority are Mexicans, that would deal with the problem of Mexico. Mexico does not have to have a functioning economy because it can always dump its workers here. There is no penalty to Mexico: we don't make Mexico pay any of the costs or take any responsibility.
I think that's the whole point of the guest worker program, which would mean that employers would have to pay these things for all employees.
Incidentally, many of the employers do pay these things, because most illegals do not walk in and say, hi, I'm illegal, wanna hire me? They have papers. All forged, but papers nonetheless, which gives the employer a cover (even though he may suspect that they're illegal). I'm not talking about day-laborers, but about people who have steady jobs in packing plants, for example, or other work of that type. However, illegals don't dare quit or ask for a raise or annoy their employer in any way because they may get caught, which means that the employer can get away with paying them as little as legally possible, even though he may be paying all the employment taxes, etc.
I think a guest worker program would pressure employers to increase wages because they wouldn't have such a dependable supply of people who didn't dare leave or complain.