I think you are misunderstanding my point - yes, having it on the GCD is detrimental to the usefulness of the skill. It is that way because it is a powerful skill. Theorycrafters have all but proven PLD is (right now, with current knowledge) superior in single target damage and overall tanking ability. I just feel like you are asking for too much. You are already the best tank, but you want to be able to stun/interrupt without any sort of reduction in any of your other abilities? We are tanks by occupation, we are going to need to sacrifice some of our abilities to fill an alternate role.
I think it is very balanced and they made this well. They made it difficult (not impossible) to tank Ifrit and stun at the same time, so that there is actually a use for your second tank - why do you think they have 2 tank slots in these things? As far as it being frustrating...it supposed to be frustrating to fill two roles at once. They want you to work together, not be so powerful that you can fill two roles with ease.
PS - we're talking about Shield Swipe, but RE: your "skillful play" comment. It IS skillful play because you need to know when not to spam your GCD stuff because you need to be reactive to being able to stun. It takes the skill OUT of it if you can do it whenever you want.
