With this logic you should get hit every time someone jumps and tries to swing at you but gets stabbed a fraction of a second before he hits you. You know, all those guys with the two handed swords and axes and so on 
Interrupting the enemy by hitting him first is an important part of the game and how melee works in M&B. You shouldn't remove this. And as rider and horse act as one being (due to both being controlled by the player) it is only fair to interrupt riders if you make their horse rear. Remember, I was only talking about rearing, not hitting it in general.
One reason less to try to attack cav? 
My first suggestion I think would be nice across the board (not just in the instance I described), but the engine is most likely too primitive to be able to decide if the guy hitting you was enough to prevent you from your swing from connecting, or make your swing weaker, or make it so it basically didn't interrupt you at all. Just a pipe dream, maybe M&B2.
Also my 2nd point was more in jest than anything, I see so many people complaining about cavalry lately and wanted to point out there is still some things that should probably be in the game but aren't. so if you don't like cavalry, just thank your lucky stars that the dead flying horses "disappear" as soon as they die.
I think the reason cavalry seems so strong is because ground troops aren't able to use very tightly packed formations and movements. If you're 5-10 feet away from someone I can most likely ride through and "thread the needle" killing the one guy but not getting touched by his buddy too far away. That and the fact that you can't pound stakes into the ground, and you can't just brace a bunch of long spears in front of your infantry wall.
The game's certainly not perfect, but every class and play style has a counter to it's strengths. Everything has at least one weakness, and I think that people should just leave well enough alone and stop constantly tweaking shit (or asking/crying to the dev's to tweak something). Maybe we'd have less buffs and nerfs if people would learn some tactics.
