From what I've read elsewhere it seems fairly conclusive that experience doesn't affect the effectiveness of missile units, either in terms of accuracy or attack strength. It only upgrades their melee ability and morale.

This means experience is almost useless for missiles and artillery. So there's no point in keeping a gold-chevron ballista once you don't need it, just disband it and build a new one when you need it.

I believe this is different to MTW where even one chevron led to a very noticeable increase in accuracy. I remember seeing this during battles, my artillery suddenly became much more accurate as soon as it garnered enough kills to get promoted.

It raises a point about gaining experience: Towards the end of a siege, when the enemy have only 60 or so men left in the town center, my instinct would normally be to shoot them down for free with my remaining missiles, rather than lose any more men. But perhaps it would be better to send in the heavy infantry, as the experience will be very useful for them, but not for the archers.