I have mixed views on this.
While it is true that Cavalry people are really vulnerable when their horse just got taken out and they're on the ground, the engine absolutely sucks at translating that properly in a gameplay perspective.
I very rarely get to actually kill a downed horseman for several reasons, first is the obvious fact that downed horsemans exists in a different plane of reality and are usually under the map or their hitboxes are just entirely unexistant for a while, not to mention the fact that they can usually get up and block/hit you without going through the animation since it was hidden by them going up and they can actually attack or block while only at 40% of the standing animation.
Another thing is how unstable the engine is at dealing with the faux-physics of this game which will usually result in the downed horseman to get launched (Read: teleport) 30 full yards behind you and everyone else that helped in taking down the horse.
These problems, especially the first one is even worse when the horse is downed on a cliffside, since as we all know, horses are actually mountain goats in disguise, and when you kill them on a cliffside, the rider is just entirely inside the cliff, so no way to hit him.
I say this could be implemented in place of the terrible faux-physics of this game, unless the codes are magically fixed, maybe by this new engine patch from that upcoming DLC.