If someone is already here, and working, in some vague legal status, why in the world would he uproot himself, and maybe a family, leave the country, and file paperwork to see if he can get back in? Especially when he is by definition too much of a lowlife chickenshit to file for citizenship in the first place?
Well gee with a guest worker program, only the guest worker is allowed to work and his/her family is not eligible for residency and the guest worker knowing he has a permit will have his family stay home, while he works and doesn't have to worry about sneaking over the border and paying a coyote thousands of dollars to do it.
It would be much better than the mess we have now, IMO.