When you get a couch kill, there should be a chance to lose it (ie, drops to ground) in the person you just impaled. A couched lance is braced against the body of the user, and the horse is providing all the momentum behind it, through your body. It would make more sense if that occasionally meant you actually impaled someone instead of shredding through their bones with a stick designed to stab/impale. Stab kills from horseback shouldn't have this happen (at all or nearly as much) because you're pushing and pulling the spear back with your shoulder (as the animation already shows).
The chance of this should be greatest when couching heavily armored characters. If you managed to impale someone through plate-mail, the warped and sharp metal would flange inwards, and could create a friction trap on the pole which would make it much harder to pull back out than push through or snap off. Not to mention the weight of someone in heavy armor would strain the lance as well as lancer's joints if they were trying to keep the lance. Heavies also pay the most by far for their armor, so that might help make the cost more worth it for them.
This would also help balance out that heavy armor (like plate with its curvature designed to cause deflection) never causes glances or deflections on stabs/couches, even when they're coming from too shallow an angle to stick similarly dense metals into one another. Not that I'm suggesting this be put in as well, but it would make a lot more sense if people in heavy armor had to be couched straight on to do much damage, and otherwise got knocked down by glancing lances.
