With recent healer discussion and current content in the game, I'm finding it harder and harder to find warrant why this ability needs to be on such a long CD.
The only positive to it is that it's still usable through Cleric Stance and healing debuffs. Other than that, it has an extremely annoying delay in it's effect and a excessively long CD meaning you probably won't use it again for the duration of the fight.
Not to mention if a Tank is in this much danger of dying that Benediction is needed, your co-healer would most likely have popped their relevant insta heal too, resulting in Benediction being wasted in a way.
I'm not denying the ability isn't strong of course, but 5 minutes for a simple full heal when essential dignity and Lustrate can get very close to the same results and be used much more often seems overkill.