To have a fleet as fishing net is one of the reasons you should put a fleet in orbit. The secoond reason is, that when an enemy fleet arrives you can defend yous plannet. Even if its your HQ, Prison, Shipyard planet, or what ever plannet you want. And you should all so place some fighters on your world, because they all so reduce the chance of escap. When you use a fleet to move your prisoners, the one reason there are no escapes is becaus your opponent cant find them and send a rescue mission. But when you use a plannet and have the right defenses, the rescue mission will most likly be detected, and that means you have more prisoners unles the mission is preformd bij spec. forces. What can even help you some more is, that you place a jedi Master as admiral of the fleet in orbit. Or as the general on the plannet. reduces the chance of escape even more. And even when it happens that a prisoner escapes, It will return to the nearest plannet that has joined to his side. Except when he was freed by a rescue mission. then he will go to the plannet from where the mission comes.