That's a damned fine idea I think
I'd like to add that surrendering should make your army retreat a fair distance in a random direction away from the enemy as a penalty for a hasty withdrawal. Basically same way as a navy losing a battle runs away for a good bit from the site of the engagement.
It is very useful to be able to engage the enemy on your own defensive terms when they approach the city, rather than having to sally forth into their waiting arms. Defense is always at a huge advantage with the way Total War battle mechanics work, and this would even give the AI a big boost, since it would be way harder for you to trap a fairly good army with a force that is only good on defense. E.g. You approach and lay siege with an army that consisted mostly of slow spear type infantry and a mix of ranged units. A terrible army to attack with, but one that is great for defense, and if you lay siege with it, you can easily starve the enemy out or force them into a battle on your terms. An automatic sally forth option, would both be realistic and give the AI (and the player) good added strategic options. I definately hope CA thinks about adding this either now or in a future expansion.