Friedlander surveyed 242 logos or insignias of American organizations relating to health or medicine in which the caduceus or staff of Asclepius formed an integral part dating from the late 1970's to early 1980's. He found that professional associations were more likely to use the staff of Asclepius (62%) while commercial organizations were more likely to use the caduceus (76%). The exception is for hospitals, where only 37% used a staff of Asclepius versus 63% for the caduceus [but remember that US hospitals are usually commercial ventures]. Friedlander notes that while the prevalent use of the caduceus for the commercial aspects of medicine might be seen as "more-or-less appropriate", he thinks the reason is that professional associations are more likely to have a real understanding of the two symbols, whereas commercial organizations are more likely to be concerned with the visual impact a symbol will have in selling their products.
"Friedlander, Walter J. The Golden Wand of Medicine: A History of the Caduceus Symbol in Medicine." New York, Greenwood, 1992