I doubt we'll enforce and deport any better than we do now. So we will get a huge number of serfs with guest worker passes (who you naively think will obey the terms of these passes) and the same (or greater) number of illegal alien invaders who melt into ethnic communities, who we don't deport
Even I am not naive enough to think people are going to skip down to the INS office and register for the fun of it! They'll do it because it will make their lives easier: for one thing, they will be able to go back to Mexico and reenter the US legally when the season for whatever labor they do comes around. Most of the Mexican workers around here like to go back to Mexico for several months of the year.
Also, for years I worked across from a building in New York that was full of Chinese sweatshops. We always knew when the INS was around, because suddenly the windows would burst open and Chinese (or Asians of some kind) would start streaming out, running down the fire escapes, etc. Those who got caught were sent to the INS camps, which are very unpleasant places. I don't think anybody would like to risk this if there is an alternative.
As for their being serfs, that was the argument that the Dems used to destroy the guest worker program in the 1970s (because the Dems wanted to promote the Farm Workers' Union). However, if they receive legal wages, legal workplace safety protection, and can leave the job if they don't receive these things, I think they're better off than they are now.