Speaking of contemporary singers, Burl Ives did this in the 1950's. Bud and Travis did several, as did the Journeymen ("Jada, Jada"). Many folkies also liked Broadway show tunes (how many did "They Call The Wind Mariah?"). Even Joan Baez and Judy Collins have committed this apparent faux pas. I would say that the illness is widespread and no cure is in sight.