Eh. Blast Furnace in BRF required two Priests for MC. Tomb of Stungeras is known as such for a reason. Mythic Jaina first phase was once again easier if you had Priests to yeet mobs off the edge. The encounters may not require you to have certain Covenants but I can certainly see certain strategies making bosses significantly easier if you're able to use an ability to game a mechanic. And that again comes back to the opportunity cost: Is it worth it to shave 100 pulls off a boss if it requires the entire raid to commit an extra 20-40 hours a week to the game? For guilds at the top end, sure. But for everybody else they'll likely just spend more time progressing.