4 factors for my rohan force:
1) (and probably most important) size of the force. in smaller skirmishes 2-3 extra troops are more valuable than 3-4 re-rolls a turn when a battle will only last 3-4 turns. Bigger forces are easier to justify the cost to add in some banners.
2) Fight skill. If my fight is better then I will more consider purchasing a banner. Sometimes your going to roll a 1 or a 2 for the fight. It happens. But when you roll a 1 to lose to a 2 or something a re-roll saves lives and works wonders. Also works good if you have a worse fight skill, but when the other guy rolls a 6, nothing you can do
3) calvery. When you are charged, your 13 point calvery are at the same level as a 6 point orc. Ouch. So if im going to invest in 12 or more cav then I'll consider a banner.
4) Warbands. You are at 3 warbands with 36 troops and you only have a few points left. If you buy another capitain, he wont get any troops, so its a waste. So your upgrading grunts to elites, why not scold a few and take away all their weapons and make them carry a tapestry around instead?
The hardest part about a banner is the self control to keep someone out of battle to pick up the banner if the bearer is killed. another 7-15 point model doing nothing? Or losing a 25 point banner?
Just like a spear, shield, and 2-handed soldier, a banner bearer has his place. Play around with them and you will see the niche they fill. The biggest problem I find, is that we are so used to playing without them, that when you throw them in, you will roll all your battles, then face palm realizing those 3 you just lost were infront of your banner.
