I don't get the archer AI in this game. Sometimes they can be devastatingly powerful, other times completely useless.
I tend to have lots of archers while playing as Gondor in TLD. One battle I was in, I had 30 or so archers all lined up along a ridgeline, pumping out arrows as fast as they were able, with another twenty mixed archers and infantry hanging around me shooting off occasional shots as well. We were attacking a mordor orc force of about 40, and two thirds were killed before the battle turned to melee.
I come across another orc scouting party of 40. All the troops are dismounted and holding position. I get killed because I recklessly charge the orcs with my heroes. Instead of letting the battle go into autocalc, I decide to press ctrl + E to go into edit mode and watch my guys. All melee orcs are killed, leaving orc archers. My archers are set on hold position, and so don't follow the remaining orc archers. There are like 6 orc archers standing there, pumping out arrows into my archer army of 100, and not a single one of them is firing back. They seem content just to pace backwards and forwards. Every now and then, one of them does decide to shoot, and eventually the orcs are killed.
It's like that. You occasionally get a breathtaking situation where archers will just annihilate everything else, or infuriating situations where they do sod all.
I have a hunch it's got something to do with the hold position or follow me commands. If you give them either of those two commands, they tend to not shoot much, apart from a couple daring archers. Also, if you let infantry or anyone else go in front, that also seems to stop them from shooting.