You may hate fates but at least marauders are like the best class to do them with (at least for the levels you'll need to get Mercy Stroke).
Mercy Stroke isn't necessary, but I think it's a nice addition to the toolkit. In the long term, the dps it adds is negligible but in the short term it's quite powerful.
Maybe your group is slow killing the first dreadnaught in turn 4, Mercy Stroke might make the difference.
You want extra damage in the last seconds of any of the Coil fights. It's not much overall, but it's an extra attack that's off gcd and the highest potency direct damage attack we have access to.
