I haven't allowed such mounts IMC because the players insist that knight's horses played a valuable role in combat other than carrying knights faster and letting them hit harder. (In other words, they wanted the horse to fight too.) There's nothing in the rules preventing that, and there's no easy way to account for a mount's power, even if it's not attacking. (The horse is still boosting your speed and giving you bonus damage with charge attacks.)
In the example where my PC was dismounted and using the horse as a meatshield, I had only done that because my PC was by himself and was going to die! I had actually told the DM (when convincing them to give me a horse) that I'd only use it the way it was meant to (eg use it for charging attacks) but got stuck in a situation where that was useless. That's not something I would want to deal with as a DM if I were running that game.
Companion rules are almost good, but a horse "companion" is effectively under the rider's control, making that PC more powerful than others (unless they all have mounts, which I don't want to have to deal with).
There's a logistics angle, but I was mostly interested in something easy. It's a lot easier to do this on the DM's side. If a group of goblins riding dire wolves attack the PCs, the dire wolves aren't just "knight carriers" but active combatants, so they're worth their full XP along with the goblins. Their healing surges are pretty pointless; either they win, escape or die. If you're dealing with the PCs buying mounts, there's lots of logistics you always have to take into account, there's XP budget issues (or not), training issues (or not - I've never ridden a warhorse and don't know how vicious or fearless they are), etc.
