The more MP you get, the higher the cost will go though. At i178 I have 5204 base mana and Clemency has a cost of 2121. I can still only cast twice before I have to start with the Riot Blade spam, which is kinda slow. I heal myself for a base amount of around 4.1k without any buffs. I've seen a crit as high as 6.5k. It has a huge drawback in the form of the 3 second cast time, though. In party content, I mostly end up using it if I see that the healer is falling behind or if several people get hit hard all at once. I can see how it might be useful to proc your own Divine Veil for some mitigation on nearby party members. Sometimes it ends up being an overheal, but overhealing is definitely better than dead party members. It's also a decent ability to hit right after a tank buster has cast since even bosses don't usually chain skills back to back. It's an extremely potent solo skill, to be sure.
With all that said, I don't see a Paladin replacing a healer. The mana cost is simply too steep.