I agree with dt. I think Michael brought back Tom because he was not a bad consigliere. After Vito's death it was his choice. At the funeral Michael says having him not around when during the planning of the massacre was a luxury that he could not afford any more. And Tom survived in the movies and the book (yes I know he is dead by GF3 but it was because of the actor). I think Michael would have killed him in an instant if he had doubts about how he worked.