On most boss fights, you'll be lucky to use Mercy Stroke more than once. Yeah sure, it's high potency and off the GCD, but taking up an entire cross skill slot for what is most likely ONE attack during a fight, two if you're lucky? Eh, I rather put Mantra in there if you got it. Yeah yeah, Mantra cross skill is pretty underwhelming at 5%. But it's a group buff, can be used on the entire party in AOE gather situations and/or used on cooldown asap which will be always useful for healers since as melee DPS, you will almost always be right next to the main/off tank.