A guest worker program can legally facilitate more workers from outside the United States. Outside workers from Mexico and other Latin American countries south of the border may not necessarily want to emigrate to the U.S. Some may not have the ability to do so. This would allow such workers to enter the U.S. and work as guest workers to feed their families back home.
However, a guest worker program will not curb illegal immigration because both U.S. employers and immigrant labor still have a lot more to gain from illegal immigration than they do from a legal guest worker process.
U.S. employers employ illegals because it saves them money. Formally hiring a worker involves various costly legal processes, such as taxes, insurance and so on. Plus, Federal laws require that any registered employee must be paid a minimum wage. Companies that employ illegals can pay them a much lesser wage than the legally required minimum wage, which the illegals gladly accept.
Most illegals carry no worker identification for obvious reasons, and businesses don't go out of their way to keep detailed legal records of the illegals they employ for obvious reasons. Thus there is no way for the government to track them. This allows businesses to hide their usage and pay of illegal labor when reporting income and expenses for taxes.
Since illegals come from a place with depressed wages, many don't understand the value of their labor, and many don't care: the lesser wage is still far more than they were capable of earning in their homeland. They often live in a cooperative housing situation to keep living expenses low, and send any savings back home to their families.
Many illegals lack the ability to formally emigrate or visit the country. Some aren't legally allowed for various reasons. Even if legally entitled, many lack formal education and the administrative red tape is too complicated for them to work through. It's far easier to hop the border and perform labor for a business that employs illegals, than to go through the legal process.
Creating a guest worker program won't solve the U.S. illegal immigration problem because, ultimately, the process still presents a maze of red tape that many potential illegals won't be able to get through. And businesses that employ illegals aren't going to turn and employ legal guest workers, because it's still cheaper and easier for them to employ illegals, which remain available in droves.