Another (probably vain) attempt to think of something you guys haven’t…
Since archers seem quite hard to use effectively, perhaps another possible benefit of using them could be that even missed shots have some impact on the morale of nearby enemies (for those factions that use morale).
(Maybe archers seem hard to use effectively to me because I’m just not very good at it, but I seemed to do better in the combat demo when I didn’t bother with archers at all!)