If the lure of illegally crossing is great enough, NO BORDER SECURITY will stop them from coming. We can't afford to search every single truck, car, and train that crosses the border. We can't deploy people every 300 yards across the entire border, which we would have to do even with a super-fence, just to make sure people weren't using ladders and other devices to get over the fence.
The guest worker program removes the incentive for spending a lot of money breaking in. In exchange, we give some people jobs, but get to tag them and make sure they leave at the end.
How? Guest workers on the books will be much more expensive than true illegals so the demand for pure illegals will still be there.
Maybe we can't search every single one, but what are the chances that they always, always, always find my third bottle of Kahlua, every single time, and smash it?
From my anecdotal POV, it works.
"In exchange, we give some people jobs, but get to tag them and make sure they leave at the end."
Make sure they leave at the end? Surely your not serious. Exactly how would you propose we would do this?
A "guest worker program" achieves that exactly how? What is the incentive to the "guest worker" for getting into the program?
Fining employers who hire illegals; now THAT removes the incentive for breaking in.