it's obviously for if you are not currently riding the mount and are AFK / tabbed out / watching a movie and hear yourself getting hit to stop a player simply clicking on the horse and escaping 100% of the time.
mount hp is not useless, it already stacks with your armour and CC and is ridiculous.
People on high tier Ox's will literally circle you and laugh because they know they're not going to get taken down. I've seen this multiple times while running with a group of 3-4..it gets old, lol. Trying usually just ends up as a waist of resources/time.