I agree with the original poster, this is really dissapointing.
I would have preferred a series of missions (at least 3 per rank) required to raise in rank. The lowest rank you could do solo, the next rank would require 2 players to accomplish, the next 4 players. To attain the highest rank you would need a full party of 8 players to complete the final set of missions.
As a bonus and incentive for players who have already completed the missions to help out players who haven't, they would receive a significant amount of seals upon completion. You would have situations like a Storm Sergeant Third Class for example leading a group of Storm Private Second Class just like they would in a real life military scenario. Neat!
This would be fun and awesome, why not implement this?
The missions could be easy to create:
1) Kill 100 enemies in this area
2) Instanced raid (turn Darkhold into grand company mission, who does this anymore anyway?)
3) Defeat certain NM's
4) Defend the city from an attack (cool!)
5) Retrieve a rare medicine from a remote area to cure the princess
6) Submit a set of spiritbonded military issue equipment
7) Attune yourself to 4 specific aetherytes that are very challenging to reach
8) Attune yourself to all the aetherytes in your cities zone
9) Protect a caravan from your city to another
10) Deliver an important message by chocobo from one city to another in a certain amount of time
Implementing the grand companies is a good idea, but to make it a great idea, make it resemble real life. Soldiers are promoted by their action on the battlefiled not by speding military credits.
There is a lot of potential here, please take advantage of it. Making you feel part of the militaries in Eorzea could be a huge and immersive part of this game. Check out the anime series "Full Metal Alchemist" for a neat portrayal of a fictional military sytem. Also, the faction quest lines from "The Elder Scrolls V: Skyrim" does a great job portraying the advancement of a character through the ranks of a military system.