If we allow mexico to join this will allow for better resource managing in mexico reduce the debt to gdp percentage allow the US to tackle the drug cartels and reduce violence in mexico.
economists say that mexico has a bright future this would improve the whole american economy and the US can invest in Mexico's future to further help it and help their standard of living.
Also the US has a more politically stable and less corrupt prone system meaning that corruption in mexico would naturally be lower itself.
since the US system with states and county's puts power to the people the Mexican people can still be self governing and tied to what other states laws say.