I understand it very well: I have several good friends who are in the fruit-growing business in WA. And to be perfectly honest, I don't really have a big problem with these folks coming up from Mexico to pick the fruit, prune and thin, and whatever else needs to be done. By all accounts those folks work very hard, and very well -- and at lower price than Americans are willing to work for, which is a serious business consideration for folks who're close to the margin.
At the same time, we really do need to have at least some degree of positive control over who gets to cross the border and work here, which is why I think some version of the "guest worker" program, coupled with a compliance-checking/registration system on the part of employers.
I think everyone would agree with your assessment. If there is a need to for guess-workers, let's do a full study on the needs and let's "invite them" (no one is forced to) to work for a period of time, but let's make it all LEGAL. Also, the opportunity should be offered to different countries, not just Mexico.