I too disagree with the playres purchasing further promotoin... However consider this.
The Promotion aspect in both ICON and CODA are under advantages... Both of which require GM approval before a playre can purchase. So you are quite correct, a player can buy it with XP, but only when the Narrator feels it has been earnt.
I still run ICON, and I still disagree with this, which is why Promotion, Department Head and Commendatoins can only be purchased during character creation. As such those advancements are player rewards, In-Character, for actions taken above and beyond the call...
I also run an active promotion system that isn't based of department heads or ranks, but instead on expanded responsibilities and junior crewmen under the players direct commmand... Positions such as Officer of the Watch, deputy depatrment heads (1 per extra shift), Team leaders and Mission Specialists really carry weight in my universe. As it is possible to receive these promotions almost weekly (with little or no XP cost), while advancing in rank has to be far slower in advancement, as nobody will get promoted to captain within a couple of months of graduating Academy. 
There was a thread about real-world navy ranks... And it seems that by far the most common ranks are those of Lieutenant and Lt. Commander, so it seems reasonable to hold little or no bar about advancing to that point for the standard player character, and quicker for the far more common exceptional player character... Its rising above that to full Commander and Captain, or higher, that should be a little more difficult.
As an aside, and a rule of thumb, I also dont require that a player character matches the required renown levels for their rank (starting at Lt. Commander) during character generation, however they are unlikely to advance further in rank during game time, until they build their renown as per the guidelines in the rules. This I feel represents officers that are relatively unknown... I say unlikely, as if they stick with it long enough, any amount of promotion is possible... Just dont expect it to happen quickly... 
All of that said, as Narrator, I reserve the right to ignore even my own guidelines when it comes to promotion and placement... After all its my game and i the NPC chief engineer dies and there is a capable Engineer that has been taking a secondary role... I see no reason why the CO might not sponsor the PC rather than a new NPC... There has to be perks of being a player... even if that means you beat the 'official' system...
DanG/Darth Gurden
The Voice of Reason and Sith Lord
“Putting the FUNK! back into Dysfunctional!”
Coming soon. The USS Ganymede NCC-80107
"Ad astrae per scientia" (To the stars through knowledge)
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