It's all about getting the work done cheaper. There's never a number that the employer has to get -- they're just opportunistically fishing for dirt-cheap wages among desperate people who weren't making it at home because of domestic mismanagement and because their native country's well-to-do enjoy effective control of their labor market and are positively administering and assigning poverty within their society for their own benefit.
IMO, I see nothing wrong with that. I lean a little more to the right in the free market. These jobs are taken with free will. However, I could see it getting out of hand if ALL jobs were competitive with foriegn nationals who were willing to make less than the average American.
So of course this becomes an issue of whether we are willing to accepting rising costs on every cheap product that we take for granted. I don't have an answer for it. I'm not willing to buy lettuce at $3 a head. If a foreign nation produces lettuce with cheap labor like we do, and we kick all illegal workers out, that insudtry will no longer be a competitive American business and we will TOTALLY lose out. I'm not sure what it hurts to have these people work here as long as they obey our laws.