Unlike in RTW and M2TW, casualties caused by missile fire add an additional morale penalty to the unit affected. Therefore it´s always useful to have at least some ranged units on the field.
Archers have another advantage: their fast rate of fire depletes their ammo fast enough to retreat them off the field and replace them either by more archers or with melee infantry. The same doesn´t work as well with crossbow and arbalests, due to their slower rate of fire.