Would tubofan aircraft benefit from adjustable fan blades?
Both turboprops and turbofans want their blades to be as long as they can be, in all circumstances: that way the largest amount of air is accelerated, and a given amount of thrust created with the highest efficiency. It never pays to reduce the bypass ratio of a subsonic turbofan.

Turbofans with adjustable blade pitch would make sense though, like the constant speed propellers that turboprop engines already have. This article from Flight Magazine 1973 gives the advantages of a high bypass variable pitch turbofan:


  
  Inherently low noise levels because of high bypass ratio and low fan
  tip speeds. 
  Rapid thrust response, allowing thrust modulation for
  accurate rate of descent control on the approach and maximum use of
  reverse thrust during the landing run. 
  Reverse thrust available
  without the weight or maintenance penalty associated with reversers
  and spoilers. 
  High ratio of take-off to cruise thrust well matched to
  the needs of short-range, Stol aircraft. 
  Good cycle efficiency and specific fuel consumption. 
  Wide engine operating margins. 
  Provides many of the advantages of the turboprop while avoiding vibration,
  ground and fuselage clearance problems and allowing better matching
  with higher cruise speeds.