The commentary after 250.116 sums up my opinion.
Because metal siding on buildings is not electrical equipment, it is outside the scope of the Code [see 90.2(A)]. Therefore, the Code cannot require that it be grounded. Quite often, however, luminaires, signs, or receptacles are installed on buildings with metal siding that could become energized. Grounding of metal siding reduces the risk of shock to persons who may come in contact with the siding.
My take was always that if the attached equipment was electricaly connected to the metal material it was bonded. In the case of a roof I would feel the same way. If equipment is installed up there, bond the roof to the EGC of the equipment. It would usually be hard not to if the equipment is metal.
