My opinion is that the clipless don't improve efficiency over a long distance. What they do offer is the ability to work the pedals with more muscles, and therefore increased power output, for short anerobic efforts. Good for accelerating, a short steep climb, or sprinting. The strongest muscles can be used without locking the feet to the pedals and to keep these muscles putting out at maximum is more than the aerobic system (at least mine) can handle anyway.