Kind of a questionof how closely one wants to simulate a real battle, vs. keeping certain gameplay mechanics.
Are you refering to gladiator? Where the Germans just stood there under heavy fire? The defenders said sure we'll take the low ground. The germans saying sure we'll let you attack us from an entrenched position.

Now think of it the opposite way around your besieging and have a sally against you. Would you just stand there and get senselesly shot up. No you would fallback and say come and get me.

Now just exploit this feature/bug for now as for when the patch comes out I'm almost sure this will be addressed that the A.I. besiegers will stay out of range from the walls on a sally, at least for stonewalls.

It's your game, play it the way you would like but asking for it to be cheasier is just plain senseless IMO.

Sure the first time I saw 1000 Gauls go for a swim was a good laugh, after that it's not funny anymore.

I don't mind the A.I. bumrushing a breach in the wall but when I have archers that dictate otherwise they need to backoff and let me capture the walls.

Now I am assuming you are on easy/easy dificulty right? Since you seem to want evry possible exploit there is.