Although its not as good as spinning slash, I feel that with a low dmg multi-hit greatsword it becomes well worth it.
With a good VIT set and MAB I can see a good competitor to spinning slash. The modifier is heavily impacted on Vit. I think it's funny that I had to dust off the old VIT gear to see what it does.
With a basic vit build (Koenig schaller, Vit rings, Af body+1, relic gloves etc) and some basic MAB items, I can do a 606 weapons skill everytime(Not counting resists) w/ a dmg 41 greatsword(Salamand Sword magian trial for drk).
Oh also, /rdm adds about 150dmg in MAB to the WS as well. 606 x 1.24 = 751.44 which is what I usually get on the weapon skill with MAB II.
Only really been using it to finish trials. No time yet to do it with abyssea lvl or a T+ mob.
I am wondering if anyone who has full koenig and novio would be able to test it out to see if they can get a sweet build going with Mercurial Sword.
I feel this weapons skill is like primal rend is to bst. Which is nice with Richardet's axe.