Is there any legitimate reason for adding momentary power when descending at idle with full carburettor heat on?
straight out of the Airplane Flying Handbook, pp 9-4


  Operating the engine at idle speed for any prolonged period 
  during the glide may result in excessive engine cooling, spark 
  plug fouling, or carburetor ice. To assist in avoiding these 
  issues, the throttle should be periodically advanced to normal 
  cruise power and sustained for a few seconds.


This procedure is common during emergency descents or steep spirals to prevent the engine from cooling off too much.  If the engine mixture is excessively rich, spark plug fouling can occur.  Fouling can be cleaned off the plugs by running the engine at Cruise power settings with a properly leaned mixture in about 10 or so seconds.