Appropriate times to use Clemency?

[Reinha]

So I ran a dungeon today.
...
I didn’t mean to insult him. I just wanted to make it easier.

Dungeons don't need to be any easier to heal than they already are. The healer is probably waiting for your HP to drop so he or she can use an oGCD heal and that free heal goes to waste along with your own dps whenever you cast heals on yourself.

It's best to use when you can't hit enemies, for preventing a death or to trigger your AoE shield.