In the Bayeux campaign I massed over 100 longbowmen. If you pay attention, all your longbowmen will target the single enemy, super-over shooting the poor guy. In AoE II the target selection on patrol or attack-move is different, with an end result that is far more efficient by acquiring multiple targets.
So what does the player have to do? Manually select small groups of units and target manually. This is a terrible, terrible thing to have to do constantly.