Of course the pikes should be grounded and the momentum of the horse would be absorbed as it skewered itself and likely wouldn't reach the pikeman.
But sorry OP, there are just too many reasons to say no (even though I can't be bothered to list them).
Yes, it would be nice if you could brace spears and lances into the ground, the horse would never reach the infantry person then. However, when an arrow kills the horse, or someone swings and breaks the legs and the horse flies into 2 infantry, it would severely fuck them up.
There are soooooooo many things that could be done to make the game more realistic, the point I (and I think Digglez) was making, is that it's futile to complain about one "realism" aspect and ignore all the others, especially in direct relation to the thing you're talking about (horses).
You want realism, every time your horse is hit there should be a chance that it rides around uncontrollably, or rears up and tries to buck you from the horse (maybe create a mini-game like in Red Dead Redemption where your horse rears and you have to stay balanced).
There is a lot missing that could make it more realistic, I think the mod is limited by the game.