It allows for more variety and flexibility than the generic ship captain components.
Make them pay through the nose for that Fleet Admiral's training or for a much smaller bonus train a large number of Captains and distribute them about the galaxy as they are needed. It allows for mobility on the part of your officers. They can be sent where they are needed and redistributed later. Invading a planet? Well don't forget your General. Your Admiral too important to risk on a ship? Then leave him on a nearby planet to coordinate your attacks. The idea definitely has merit. It could potentially add an entire new layer to ship training. Let's say you have a fleet where each ship has a well trained captain verses an identical fleet with no trained officers. One guess as to who wins.
