As said, per GCD in a big pull total eclipse is 110 potency per mob and is the only AoE GCD move PLD has access to. Healers have stronger spammable AoE GCDs than this, WHM has Holy and aero 3, AST has gravity and even SCH has miasma 2 and baneable DoTs. If your PLD is using clemency, use that time to DPS.

In single target yes a PLD should rarely use clemency other than maybe to proc their divine veil or if the healer is busy/Dead.