I play vs hard neutrals, my troops become more wounded after each battle. Having weaker units means I will need to spend more time on healing. I don't care about MP where you set weak neutrals to be able to use autocombat, that's a different game with different combat, mounts are more powerful in such game indeedYou're missing the fact that this is about strategic map mobility and the huge advantage you get from clearing the map faster.
40% damage won't do anything if you're not even in combat, nor will it help it you are unable to reach the units before dying.
I suppose I should've prefaced this with the fact that you should play against human opponents to understand the power of mounts.