Well, they're only good for cargo. A big box without even access between the ship and itself. If you want to do anything with it other than pickup or delivery, you'd need a hull.
The clamp ship can act as a ferry or a tender.
There probably should be some rules about external cargo and incoming damage. Maybe just make it that Cargo hits don't benefit from hull armour? I could see there not being much difference between cargo held in an external box and one sitting behind an unarmoured hull anyway.