not a good comparison.. clemency has a long cast time (3 seconds) and on normal cases healers (faster heals) get to top you off (or any party member) before your heal lands.. warriors use equilibrium for o-shit moments when heals are needed most (after a spike / tank buster). yes you can precast clemency.. that is if you're lucky enough not to get interrupted

im not 60 yet but i think the most ideal situation for clemency to get really used is when off tanking to help lighten the healing load. then again it costs more than a third of your mana bar so you really wont be able to spam it anyways

equilibrium (for tanking)
clemency (for offtanking)