Imagine that Defiance prevented you from using Inner Release. Or that Shield Oath prevented you from using Fight or Flight. What's more, imagine that switching stance mid-window terminated your offensive buffs.
That's basically what it's like to tank with Grit.
The nice thing about Defiance is that, through Unchained, you can gain the mitigation and enmity benefits of the stance for a brief time without any of its drawbacks. Consider this: if you removed tank stances, and instead had an Unchained style cooldown which gave you some sort of a defensive boost and your attacks increased enmity for its duration, you''d have essentially the same functionality that WAR has now.
You could even make it thematic. I've always felt that Bulwark was a bit lackluster. You could give PLD had something that say, gave them increased enmity, a higher block rate, and reduced the recast on Shield Swipe while active. Blood Price is similarly unimpressive on DRK. You could give them something akin to
Dread Spikes.