If I could mention an "appropriate" time to use Clemency, I'd say to insta-launch your Divine Veil. If you want to prepare for something but you're still high HP, so the heal dosen't heal you (which is normal) you can use Clemency to set your Divine Veil on the team.

That's the only reason I ever used it (or if the healers are all dead and we're trying to push the last 1% of a boss lmao)