@OP
Just wanted to give a different perspective on shield oath.

Shield oath is on gcd, but it gives ehp once the skill is active. WAR can switch to defiance off gcd, but the act of switching stances does not result in an increase of ehp. To increase ehp, war needs to follow-up with a skill like innerbeast, which is on gcd.

I've always kinda seen the combo breaking mechanic as being a tradeoff for the ability to change stances whenever you want (provided adequate mp) as opposed to WAR being locked out for a cd.