I can see it being realistic that horses take SOME damage if they ran into a tin can man-at-arms (some bruising and such). Contrary to popular opinion, I am all for adding realism to the game.
HOWEVER, I would only agree with this if horse trampling was also adjusted to be more realistic...that is, potentially fatal. Every time you are bumped by a horse and actually knocked down, there should be a 25% chance of a 1,500lb hoof landing on a vital organ.
Until then, I vote no.