Cavalry already have a massive advantage due to the fact they have a press button, kill anything feature included (Lancers), they are strong on horse but when they fall off they SHOULD have a moment of disadvantage, if they can just roll away and instantly escape the death after finally taking down your horse cavalry WILL become overpowered.
Also, it's not realistic whatsoever, a horse is charging at a high speed and gets stabbed and killed by a pikeman, you've just been instantly stopped and due to the speed you were travelling at, flung forwards, if you hit the floor at that speed it's going to fucking hurt, do you seriously believe that you'd be able to roll away after you've just hit the floor at like 40 km/h.
It would be an illogical thing to add, and doesn't do anything but give cav another massive advantage other than the couched lance.
EDIT: Okay, with Gravoth's addition then I think it would be acceptable, if you fall from the horse you should take damage for sure.