United in Stormwind Cards List & Guide
Well, okay, celestial druid hard counters the quest, sure.
But other than that, it's nice to think that priest has a way to win games now other than some janky miracle combo or infinite resurrecting minions. It's not perfect, but it's something.
And it's not like it's impossible to counter, either, so there will still be some thought that goes into when you finish the quest, when you play Xyrella, if/when to play the shard.
But I think most priest decks will run this just because you generally aren't doing anything else on turn 1 anyway.