A Shield Oath additional effect where when you are attacked you heal for 25% of damage suffered is not overpowered as Dark Knights have Soul Eater, which absorbs 100% damage dealt, where the potency is 260 or 400 with Dark Arts.
My Dark Knight can easily heal itself up. Yet Paladin needs Clemency to heal up, which is probably not practical in battle when you are being attacked so the 25% damage suffered heal would help them be more comparable in terms of capability to the Dark Knight.
Besides, this effect helps the Paladin to be a more defensive tank.