Pulling out Morty from Terra I feel would leave the Traitors too weak on the ground and risk the Imperials getting a Space Port Victory. Or even if not Space Port Victory, then allowing the Imperials to take back the north-west space port where Mortarion starts at (in scenario 1). If the Imperials hold both northern ports, Traitors have to secure the southern ones, move up north and take both ports that the Imperials have had time to reinforce, which could lead to a Holdout Victory.
I like the things as they stand. As Traitor, you have to get a feel for when the Imperials are ready to launch their attack against Horus and bring him down to Terra or mess up with the Imperial plan using your own order cards, forcing them to take bury actions.