In the early-90s, when Nafta was being promoted, I had a hard time deciding whether I agreed with it or not. The good versus the bad was difficult to gauge. The argument that eventually won me over was that it was supposed to help boost certain quarters of the Mexican economy, leading to a substantial decrease in illegal immigration! Being burned (severely) on that single point, I really don’t have any inclination to find myself ever defending the darned accord, even if some of the economic data might indeed prove positive.
You need to ask yourself: if we backed out of NAFTA tomorrow, would illegal immigration slow to a trickle or even stop? Don't kid yourself.