Agreed. However, there are two things to consider. If the adapter is free of optics and only serves to restore the proper lens flange distance the lens will cast an image on the sensor plane which is substantially larger than the image sensor, producing what we call a crop factor. At the same time, the light collected by the lens but not hitting the image sensor is lost, representing a loss of aperture and loss of light compared to a native lens using the same frontal lens area to illuminate just the image sensor. Consequently, from a size, weight and cost point of view an adapted lens can not compete with a properly designed native lens. However, I agree that the image produced within the cropped area will show the same illumination as the center portion of the larger image captured with the same lens on for example an FX camera.
I wish that the Nikon 1 offered a pro quality, prime 300mm f/4 native lens.
I am looking forward to checking out an Olympus OM-D with the new native Zuiko Pro, prime 300mm f/4 lens.