Take a round of battle. You brace your spear. I assume that makes you stationary, not able to aim it, doing crazy damage to whoever runs into it. What cav would ever run into a stationary infantryman with a spear? With the maneuver of most horses cav can turn themselves to safety even when they didn't really see you until the last moment. That is best case scenario. The scenario I expect is, they see you standing there, gallop past you and stab you in the face. Or also very likely, another cavalry guy lances you in the back while you are stuck facing one side. This isn't Total War where a line of 200 spearmen can cover a flank, this is fighting with 100 players on a server, spread out over a big area, where cav are able to go wherever they want, whenever they want. Not even to mention the ranged target that you will be
Seriously, how do you figure it would have any use at all? Bracing a polearm would reduce you to a very small version of wooden stakes, except that you can die.
Napoleonic wars have auto-brace spear on and you can orientate it however you like as long as you remain crouched and without moving (unrealistic but heh), so its really powerful and useful, as long as your spear lenght puts you on the safe side against the lenght of the cavalryman weapon.
I dont know why cRPG doesnt learn more from warband mods like Napoleonic Wars - Persistent World - VikingR. There is tons of battleready features around and if they were converted and adapted to cRPG the game would be greatly improved (I hope I will see more of them in MBG).
On the matter of colliding horses, I dont think there is possibilities of transforming dead horses into projectiles that would do charge damage, but one can hope we had collision damage and more interactivity between horses and the rest of the world environment (including other horses), and not
ONLY players on foot.