To the second point: I know that the Gauls used archers more for irritation than solid effect, but I forget why... probably a lack of a strong tradition of archery, due to their heavy use of javelins. No idea how much this applies to the Casse, but I would be willing to bet the same concept applies; however, the archers surely have a demoralising effect on the enemy- after all, showers of arrows coming from the enemy line aren't exactly encouraging.