A line of Sharpshooters firing in unison is indeed a wonderful thing.
Volley fire works wonders against cavalry in field battles, not entirely sure why, seeing as it theoretically lowers the firing rate, and therefore overall damage, of your crossbowmen.
I have a theory (That is mine, it is my theory *atchoo atchoo*) that it's because when firing as a volley, the sheer amount of bolts overwhelms shields, seeing as all it takes is one shot to get through and then the shield drops. But this still doesn't seem to account for the massive amounts of newly dismounted Knights wandering around after every volley.
It probably is just an illusion, created by all their horses being killed at the same time, but dammit, it's an awesome illusion.
I'll continue being happy with my placebo.