The strength of great commanders lies not in their ability to cast recruit (which is peripherally useful if, I suppose, you were in the process of being crushed by your enemies), or even build a Command Post for civs without the Org trait. It's in the buffs provided to unit types under their command.
The generic one increases your odds of prevailing in combat in an "all things being equal" scenario, and that right there is enough to make their presence worthwhile. But when you begin specializing them and assigning them unit types is when they get a little crazy.
You can successfully defend a city with walls against 20-30 attacking units with two archers under a great commander with the a couple of Defender promotions. It has really changed the way you should approach war. The command promotions available to every Bannor unit that invests in them really makes them an overwhelming military force by mid-game (when most civs only have 2 or 3 great commanders, and often one or two have died at that point).
Obviously, the best source of great commanders is battles, but once you have a few you really should start spreading them out in specialty. Having one be a general battle leader is good, and if he's promoted to the point where he can lead 7 or 8 units, those 7 or 8 units can reasonably expect to steamroll even much larger stacks of defenders. They also can somewhat alleviate the need to build disciple units to heal your armies, which can be a great help to civilizations without access to religion for higher tier medic units (Illians, Grigori).
