T&B-glad you brought up that sling training anomaly. Example: in 1963 all pilots being trained got sling training in the basic course ( H-19s or H-34s ).
In 1977-8 we trained the trainers at Ft Rucker ( the new Instructor Pilots for the UH-60 ) in slings. However on a trip down there over a year or two later, they had moved that training to the units. Maybe some senior IP can pass along the thinking associated with that move??