I agree, the frictional losses of an unloaded (a SON or Shimano is permanently engaged, with associated eddy current losses - a hub dynamo with disengagable magnets was discussed in some thread I can't find a while ago) hub dynamo are lost in the noise. But I also believe that the losses from carrying another 5kg of whatever on your bike are lost in the noise, so I accept that opinions differ greatly on such matters.
It's also fair to say that the idle losses of a spoke dynamo are going to be somewhat higher, on account of the gearbox, so the clutch becomes more useful.
It does smell a bit like vapourwear, though...