I don't know. Certainly more useful in raid situations; even in 5-man, assuming single healers, it's very limited use. It's an instant cast sure, but it still activates the GCD, meaning you've got ~8 seconds to make use of it, which means HoTs or instant cast heals unless the target has also activated some mitigating effect (Shield wall, bubble) to let you get off a massive heal. If not, by the time you cast this, wait out the GCD, and then get a large healing spell off, what ever caused you to cast it in the first place will have probably killed your target.

I suppose the large heal in that case could bring him back up from 50%, but I think it's got very little use in anything less than a 10-man raid, or situations where you know you have burst damage coming in. And even there, it's only going to get you through one of those burst damage effects, or a few extra seconds.

Drop the bonus healing to 10 or 20%, increase the duration to 20 or 30 seconds, and up the cooldown to 5 minutes to prevent raids from stacking enough priests to keep it up constantly, and I think you've got something. Or at least remove the GCD, like the Paladin judgment.