Wow they die from a two foot drop? I've been jumping off rocks in the wild all the time. I guess I better stop. Plus my bought horse always joins in the attack so whenever I see someone down the road I get off and walk to see if they are friend or foe,
It's happened to me twice. I go to drop off a small height (maybe half the height of my character) and the horse does it's "jump" animation, floats across the ground for a distance and then lands as if it just fell of the throat of the world.
It's incredibly annoying because I like to take shortcuts and stick close to mountains for cover. (In case of dragons and so that I only have to worry about normal enemies on one side of me.)
So I just don't bother using them unless I plan on sticking to roads. (It was helpful for traveling to all the Stormcloak camps as part of that questline.) That's when I used them the most.