I keep trying to rack my brains as to what possibly suggests that Dark Knights (traditionally) use Scythes. I keep coming up short. I'm not even sure why they were added as a weapon in XI, either. I always opted for Greatswords.
Because some people though that a peasant with heavy armor was somewhat cool.
They were wrong
Anime has some cool scythe-wielding characters, but in a "real" world it's not really practical.