DNC has very few slashing options since they don't get access to Savage Blade, their sword WS options are terrible and they have no other slashing weapon. So...any enemy which takes bonus slashing, or penalizes piercing/blunt. TBH their blunt sucks too, so anything that resists piercing, Onion Sword is probably your highest damage option.
Ruthless only makes Fusion with multiple WS, this is Fusion in a single WS which has different practical applications. Someone saying "Open light for my Fragmentation WS" is probably hoping you'll do it right now, not after 2 WS. What if the mob doesn't have enough HP for you to do multiple WS to set up Fusion, or if there's WS spam going off where you can't do a 2-step to open?
Other jobs can also take advantage of Fusion, since if you're not PLD/RDM, you can't open Fusion (single WS) with Sword unless you have Onion Sword.
It also allows you to do solid WS damage while not walling others (and actually setting up Light) for mobs like E/G bosses in Sortie, etc. Could avoid WS wall on Kalunga while also providing Box Steps, Haste Samba, and healing.
That's just for DNC...