The "Never fired, only dropped once" joke started with the South Vietnamese.  U.S. soldiers in Vietnam made the joke up about the ARVNs.  Just from what I have read in books, ARVN soldiers were very poorly trained, full of VC and NVA spies, unmotivated, forced into service, and very very scared.  So its kind of a grim joke.  Yank some kid off the street, hand him a rifle, throw him into the meat grinder.  As far as I know, there was no 'tour of duty' for ARVN soldiers.  I believe it was 'fight till ya die'.