Practice them as soon as you are wearing that armor type. The way I understand, every armor slot has a chance of blocking an attack, this skill makes that happen more often. I don't know the stats, or how it scales based on AC, but I just see it as 5% damage reduction versus not having the skill. Even on a giant it will perfect without any special effort. It is not almost useless without shield block. A shield is just another armor slot, like leggings, or a breastplate, that can absorb an attack in addition to the shield block skill. That additional blocking chance won't happen if you don't have the armor use skill that matches your shield material.