Am I missing something obvious?
Units shoot at the nearest enemy so it is difficult to get more than one unit to shoot at the same target.
My Persian immortals shoot at some Greeks at close range. They get 6 dice, plus 2 for superior shooters less 1 for shooting at phalanx. They are a bit lucky and get 4 hits but the Greeks probably save 2 with their armour and shields anyway. So 2 hits and no effect. My ordinary Persian infantry have even less chance and the levies need 3 hits out of 3 and all the Greek saves to fail which by my maths is about 64:1 against.
My English longbows shoot at some French knights. They are at close range so they get 8 dice, probably 4 hits. But the knights defence is 2 so I would be lucky to get even 1 hit. Again, no effect.
Even if I get 3 hits and discourage someone the marker comes off automatically at the start of the next turn anyway, so unless I am in a position to charge them in the same turn i shot them there is no point bothering at all.