There's a certain amount of elitism a staff member is entitled to, so I don't think they are required to post often. There's a kind of enigmatic presence whenever a not-so-active staff member posts, and that comes with the title.

They should be a trusted member of the community. They should also moderate, but really, who's to police that other than the other mods and admins? It's not like there's going to be a mutiny of members over it.