Its been beneficial for the higher ups of both countries. In Mexico, they get to ship their poor over the boarder and not deal with them. A large chunk of the poor stacking up at zero with no jobs tends to led to them getting angry and sometime angry enough to get into extreme politics and start attacking their government, go look up who Subcomandante Marcos is and him leading a communist revolt very recently.

With them in America, they not only get them out of their hair, they get to work jobs in America, and they send a lot of their money back to their families in Mexico. Mexico makes the majority of its export money from remittances, even more than from oil. And the American oligarchs benefit because they have a cheap labor pool which undercuts their need to actually offer protentional workers a better wage or benefits.