AO passes are supposed to be used is in Renderes, that do not support global illumination or doing so takes too much power and game engines.
It has no real place in Raytracers, but the way you are supposed to use it is to multiply it with the Diffuse pass.
That gives the illusion of properly lit details that occlude each other. In a raytracer it is a physically incorrect fake.
Of course you may use it as a dirt mask as described above.
