Another reason for Tom as consigliere: Vito envisioned his family gradually going "legit," and Tom, a lawyer and uninvolved in the muscle end of the business, was a better front than someone who came up from the ranks. Michael later wanted Tom uninvolved in The Great Massacre of 1955 because he needed Tom's legit front to pave the way for his Nevada move.

I think Tom was also helpful to Vito because, given the general prejudice against Italians, politicians (mostly Irish in NYC at that time) felt more comfortable with Tom.