You may be refering to the Ptolemy group, in which the answer is that they have to be as effetive and efficient as possible. And the GN Drive Tau, because of its limited operational time, requires a specialized recharge station to bring back to full capacity. In which, the perpetual energy of the True GN Drive makes it perfered for missions. But the same cannot be said for other parts of Celestial Being, particularly in recent side stories (Gundam 00I Chapter *, see: Fon Spaak), members of the organization have used false drives in their operations independantly of the main group, who possess all five of the original drives. I hope this was what you were looking for.