Seems to be that once you get one, there's not a whole lot of reason to keep doing them. Of course, Spooky might want to since they do drop event-specific artifacts.

My guardian (the new one, Aiven) has been grinding the dragon motes. Mostly because it's really easy in Silverwood. Have some of the artifacts, and have sold some for decent coin in the AH. Actually, everything I've put up guardian side has sold... Better economy?