Yeah, because their stats are generally crappy, the single hit and attack bonus on Y/G/K pretty much puts them well above anything else, plus the overall much higher odds they'll use one of those instead of like... Slice.
Nah, my Raka never uses Slice (and in fact, NPCs will rarely use the two lowest level WS once they have higher ones). Shadow of Death, on the other hand... she LOVES that WS and probably uses it more than Guillotine and Cross Reaper. >.<
At any rate, I use Scythe for fashion ONLY. Mandibular Sickle gives a weapon model that looks like an Ivory Scythe, pearly white. Goes great with her Opaline Dress set and Lilac Corsage. Is DD performance and SC ability better with a GK? Yeah, probably so. But when I'm using an NPC I'm usually doing farming-type stuff where it really doesn't matter much, so I go for full-on style.