Yes, the rafters need to be made of the proper size to support structural loads on their own. In the German system mentioned, the point is to resist wind loads, keeping in mind that the roofs they are used on are very steep, often steeper than 45 degrees. Essentially they are acting as another level of bracing, using the rafter to pairs to brace each other. They are also almost always placed about mid span if they are not supported in any way by purlins, and many times may serve the extra function as joists for a loft floor.
