Im not sure I would like to see wall arrows go. It already bothers me that when I stick archers on walls, their ammo runs out - surely if you are defending a city, you would get people to bring you more ammo, after all the whole infrastructure is there, not like on a battlefield where you are limited by what you can carry. Arrows from towers sort of compensate for the fact that your archers run out of ammo, but they arent quite the same. Losing that small bonus would just weigh the battle in favour of the besiegers, and I dont think cities were that easy to break into by force. Starvation seems a much more plausible strategy, which is why I always starve cities into submission when I play.