[WarBringer]

I agree with you to a degree, the firing of arrows to cause disorder and disrupt the charge slightly may indeed work, but bear in mind that orcs use crossbows, which take long to load and longer if you have enormous war horses bearing down on you. In this manner, Uruk-Hai may be able to fire one massed volley before the horses hit the front line. Still, not a bad strategy at all, especiialy of it causes the front line horses to turn about in pain/confusion and disrupt the charge.

Also, remember that retreat would be difficult in such a little amount of time.