I agree that Rocco had become dispensable the end of II. Rocco was Clemenza's protege, Neri was Michael's protege--just as Tom was Sonny's choice for a brother, not Michael's; and Tom was Vito's choice for consigliere, not Michael's.

One thing that always interested me: Rocco was completely identified with Michael--we even see him listed as a caporegime in the FBI chart at the Senate hearing. Since Rocco was killed at the airport (or if he had been captured), the police and FBI would immediately see that he was acting on Michael's orders. So (attention: Oli), we might make a case that Michael intended Rocco to escape, if for no other reason than to protect his own reputation--so recently under attack at the Senate hearing. On the other hand, Michael may have wanted Rocco killed--and indentified with him--as yet another warning to his enemies that no one gets a pass from Michael Corleone no matter what.