Clemency is only used for "Self-Preservation" mode. Meaning either one or both of your healers are dead and you need to solo heal the remainder of the bosses health or survive long enough for the healers to be revived. This happens waaaaay more often then people think though. Especially in Dungeons, Heaven on High, Eureka Hydatos etc. If you see someone die on your party list, immediately start looking at your support skills (Cover and Clemency in particular.)