Well, in my opinion it’s not a bad idea to use scepter and shield or focus (I prefer the shield) in WvW when you are defending or against a high number of enemies. But it’s true that it’s not as much damage as a melee weapon, but that’s normal. Guardian are melee characters, not ranged ones.
About the staff, it can be useful but i don’t really like it. It can be good to defend, but in general, it’s better another weapon.