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.
Granted all that, but it still has vital uses that warrants its implementation.
Guarding narrow flanks, chokepoints, bridges (you should be able to create a small area of collision when multiple polearm bracers group up, so you can't just walk up to them, however they can be shot), corners and whatnot. Cancelling a brace should also be quick, I don't think standing up and slightly altering your grip takes more than 2 seconds.
All in all, this could be a feature to spice up battle a bit (as well as siege).
Also, I think one should be able to aim it slightly. Turnspeed obviously needs to be moderated and you shouldn't be able to lower or raise it more than an inch or two.