Even if our government did implement a guest worker program, we certainly don't need 15 million Mexican guest workers to pick our produce. Also, how would we make sure that the illegal aliens who got into the guest worker program do in fact go home after their permits expire? Right now, we have no way of knowing whether a foreigner whose visa has expired has left the United States when they're supposed to.
Hispanic workers also provide housekeeping in hotels, work in meat-packing facilities, clean offices after-hours, and do construction jobs.