Mercy Stroke - Necessary for MNK?

[Aeliott]

While you can only use it when they're below 20%, it's still a free extra 200 potency to weave between cooldowns, and most of the other cross class skills are kind of unnecessary like Foresight, Keen Flurry...outside of solo'ing you shouldn't be in a position where these would be useful. Then there's Skull Sunder, who the hell thought that'd be a good skill to make cross class? :P

It's not imperative to use, but it's better to have more damage than the other "lite-defense" skills. Impulse Drive is also worth consideration, throwing it into your rotations can help get those last couple of seconds out of Demolish sometimes, and from the rear it's 180 potency, so it doesn't slow down your dps. I never used to use it, but I've grown to consider it more useful than Bloodbath, which is ultimately just another minor defensive skill.

[Shirai]

Impulse Drive is also worth consideration, throwing it into your rotations can help get those last couple of seconds out of Demolish sometimes, and from the rear it's 180 potency, so it doesn't slow down your dps. I never used to use it, but I've grown to consider it more useful than Bloodbath.

Personally I've started loving Impulse drive and have started using it quite regularly in my own rotations.

As for Mercy stroke; While not necessary it is indeed a pretty potent off gcd direct damage ability.