Most people crossing the border now are young men looking to work hard, earn money and return to their country. With the 6-month visa the farmers will have their labor during peak months, Mexican men will be able to return to their families and the country that desperately needs them, and both countries can better monitor and control movement across borders.
This is not the Bracero program, where immigrants are tied to a single employer who often mistreats them. These workers would be able to move freely between jobs during their six months. This is what many Mexican migrants want, this is what farmers want, and this option should be offered in place of citizenship for those here without family ties and who don't want to pursue citizenship.