It would make life interesting if there was an element of post-battle "prisoner exchange" whereby, if you'd captured 1000 men but lost and they'd only captured 300 of yours, then you get all your men back, for free, by means of exchange but they get all theirs back free too, because they won and this is 'victor's rights'.

However, I believe the principle of it is that your men can't rout off the field and guard captives (ie make them flee as well) simultaneously, so they are deemed to have escaped your clutches as soon as your own men run. Therefore, prisoner exchange is not an option. They get all their men back and you must pay for yours.

Basically, if you've lost, you may as well hit the button, so you don't have to face them again. They will be able to attack again the following turn, whilst your returned captives can't be commanded until the year after next and frequently get returned to a province other than the one from which they headed into battle, forcing you to make some hasty troop moves to set up a defence.