In answering a previous question I feel Tom was left out of the family business for the exact reason Michael said. He told Tom after he had been shot at "that is why you are the only one I can completly trust right now" If you remember Don Vito had said to Tom "I advised Michael on this decision" The Don knew Michael would run into trouble as he grew in power and by having Tom not involved in the family business, he could completly trust him in the event of an attack on Michael. Atleast thats what I got from it.