Agree with Leshma there.
Notice it two times already today, once with an archer.
The fact that we could hit them on the ground was a good way to balance their uberness on their horse, if you take that away, because the horse sucks hit, then add fall damage.
Though i'd prefer if dead horses don't suck hits like dead human bodies. Dunno if that's possible, or if the two are "bodies" and thus have the same features.