I think it has to do with what the stances do, and that's why they make it gcd, break combo, and cost mp. Defiance increases health, but when activated does not restore the difference in health, this is why the war stance dance is how it is. Swapping to your tank stance results in a 25% gap to max hp, that needs healed. Shield oath is perma rampart, activate it and you instantly have 20% mitigation. I swear I read a dev statement that compared shield oath to a CD as it reduces damage, no drawbacks to switching back and forth and so a fear of it being op arose. Now, for my opinion: clemency should be instacast or uninterruptible, I don't know how else to say it. Shield oath and sword oath, no longer breaks combo, still gcd. I don't feel that doing this would be game breaking, but will instead make a wonderful class even more fun to play.