I think I know where you're going with this, MC:
Remember that Michael said twice (near the end of GF and just before the Johnny Ola meeting in II) that Tom was just his lawyer and handled only "certain parts" of the family business. It was his attempt to keep Tom focused on the "legitimate" side of the family business. Therefore, who better than Tom to represent Michael as a "legitimate" guy wrongly accused by the Senate subcommittee?
Mafiosi have traditionally used "outside" lawyers to defend them--lawyers who were on retainer to them, not made guys or associates of their crime families who were connected with the day to day business of the mob. Those outside lawyers had other clients, which helped maintain their credibility when defending Mafia guys. For example, Roy Cohn, a powerhouse, politically connected NYC lawyer, defended Gotti when he was nailed for killing James McBratney for the kidnapping and murder of Carlo Gambino's nephew.
The case plawrence referred to was when a NYC judge disqualified Bruce Cutler, Gotti's long-time lawyer, because FBI surveillance tapes caught Cutler counseling Gotti on regular Mafia business, not just the case he was being tried for (the Castellano killing). The judge said Cutler was acting as "house counsel," which in legal terms meant that he he lied when he said he was simply representing Gotti as a client. Cutler could have made a stink about it, but then the Feds probably would have come back at him and indicted him as an accessory to some of the crimes they caught on tape. I think they were firing a shot across Cutler's bow.