I think there's a difference between knowledge and skill that people aren't taking into account. I'd count knowledge as knowing what to do and skill is what it takes to execute it. So an engineer can "know" where to put his sentry nest etc in TF2 however does not need a lot of skill to execute construction. Whereas a sniper may know the best spots to camp and know which targets are a high priority but still needs skill to execute the shots necessary to be successful.
Of course engineers require skill as well but not to do their main job of building things.
Of course engineers require skill as well but not to do their main job of building things.