I do find it stupid for the legs to be uber-vulnerable simply because that the heavier, slower more armoured horses are actually the ones easier to hit in the legs and basically avoid the armour altogether, thus really in the long run a cheaper, faster, more manueverable horse is superior.
If anything I think the Arabian Warhorse is OP, well when it is +3 loomed more so, it's ridiculously maneuverable for the speed it also moves at, it can turn on a dime and be back to full speed in a second, yet it's cheaper than any heavy horse, including the destrier, it's far superior for melee cav than armoured horses right now.
Sure it takes less damage than the bigger horses at range, but if you are aware of anyone aiming at you it's so easy to dodge shots with it. In melee with anything hitting your horses legs, it negates any worth of a heavy horse and automatically makes the better choice the Arabian, because you are far more likely to avoid that situation or keep range from an enemy.