Sub PLD is ok on certain jobs, namely RDM for solo / utility in low man situations but on DRK... just don't see it happening. I can see /RDM for specific situations, even /SCH if goofing on missions / farming stuff with friends.
For GS, well Sickle Moon is ~ok~ at level 60, Guily is still better IMO but that is a different debate. Spinning Slash is made of win vs anything 10 levels or more above you, but fails to Guily on anything less then that.
It is a decent WS at 60 yes, certainly the best until Spinning Slash.
The thing is, this person was 75. :/
With a GS skill of approximately level 60-65
Anyway, Spinning Slash doesn't fail to Guilly on anything less than that, Spinning Slash is an excellent WS if you have the right build for it. Anything lower level than you maybe, but it doesn't need 10+ - unless you're talking about Souleater Guilly or something.
Even on weakish stuff, Spinning Slash can bring in some nice consistent numbers, it'll probably average below Guilly but I wouldn't say it fails. The problem with greatsword is the greatsword itself, they just aren't as good weapons as Scythes.