Please also remember that irregular archers won't do wonders, they are not like Elven archers in Warhammer: Dark Omen at all
Even the elite ones wont bring you victory on their own (horse archers are another story however, they can because of their mobility). However sometimes even the poorest archers give satisfactory results, try them on poorly armored militia cavalry locked in melee, for example.
To summarize, they are mainly support units that wound and sap morale greatly. In most cases enemy unit that was under fire will rout in melee quickly because of wounds and morale loss. Here each regiment has their own morale level.