Flaming arrows reduce morale and at least give a chance to set stuff alight.In the case of sub-par archers (peasant,militia etc) It at least is more effective than just a regular arrow inasmuch as it CAN in limited cases either reduce morale OR set fires.

Have pics at home of a field battle (not a siege) in which there are 2 trebs that were set alight from the volume of fire-arrows being shot. Quite the sight...and in that case those are arrows that have "missed" their targets and hit the trebs instead since the treb unit was engaged in melee. Sooo ...some slim chance at least of success youd think.

Ive only noticed a tower unit massing pre-attack for the 1st unit, the others behind it simply climb up and file out, so unless its a hvy inf or something decent going in as the 1st unit,the advantages are few over speed,manuverability and the option to pick up the ladder and reuse it. Towers as well can get burned down whereas a ladder unit must lose all 3 ladders 1 at a time in order to become ineffective.

I do agree that if you have sufficient forces you should try to group as many
archers as possible to concentrate their fire. However, it can be said that having 2 groupsis superior to 1 since there are 2 flanks presented to attack.