Some promotions trickle all the way down (chain of command, enlisted) while others do not (Leadership).
I've been running some tests and crunching some numbers in excel to try and figure out some way to balance things out a bit.
Currently I feel there is too great a gap between the experience gain of Bannor Captains and other civs Great Commanders. I believe the man driver of this gap is the Regimental Discipline promotion and the way it stacks. While you have to give up a promotion in order to get RD, Battle Command I is all most as good. The Bannor also have the option of a Chrismatic leader to make getting those couple extra levels much easier.
For example;
CASE 1:
Great Commander (level 1, no promotions) with 3 attached Warriors (Level 2, Combat 1) fighting 3 Barbarian Warriors (Level 1, no promotions)
Combat Odds: 73%
Experience to Warrior for winning: 5
Experience to Great Commander per victory: 1.25
CASE 2:
Captain (level 1, Field Commander and Battle Commander I) with 3 attached Warriors (Level 2, Corporal).
Combat Odds: 70.6%
Experience to Warrior for winning: 7.81
Experience to Captain per victory: 4.29
Reducing the experience bonus gained from Regimental Discipline from +10% to +5% would lower the Captains experience per victory to 3.12 while lowering it further to 2.5% (which is still a 10% increase over the base 25% experience gain) would lower it to 2.54, still double that of the standard Great Commander.