Migrant workers migrate for the ability to provide better care, financially, for their families. Forcing the families to stay apart is cruel and unjust. Further, the more people they reunite with funnels more money into the market. Each of those family members must have food, a place to live, and transportation, which pours money into the market. In most cases migrant workers provide more to an economy than they use.
Families should be allowed to stay together as a basic unit in human culture. Sometimes migrant workers don't know they are breaking the law before they are deported. Life as a migrant worker is hard enough--families should stay together for an extra sense of security. Families shouldn't have to be broken up unless absolutely necessary, i.e. if someone's life is in danger.