I think the practicality of doing this was to stop both sides losing gear/casualties. One side would be victor, but both sides get to keep their ticks (-1 for losing side
), stuff (with losing side leaving a town/castle if its a siege for example).
Saying that.. a losing attacker could just re-seige straight away, not sure how to get around that without severe and unpopular limitations... Maybe Ive read too much Game of Thrones..