Its not nearly as strong as you are making it out to be its NOTHING compared to the night and day difference between getting Dark Intent or not (which thank god is getting removed as a single buff). And getting a single warrior in your raid is a lot easier than getting DI from a warlock was. But if for some reason you cant link with a Warrior cause one isnt there or cause you need something else for a specific fight your dps wont be that different really, its a 5M CD it isnt that drastic. As a balance druid it would prob be smart to macro Barkskin in with Recklessness to negate the damage taken component.
Its because he noticed that DPS specs were getting healing spells (Mage/Warlock) and having the shadow priest get tranq as an optional healing CD made much more sense than a healer getting it balance wise