"There is nothing in Traditional Folksong to compare with prescriptive right-on proslytisation as purveyed by Billy Bragg, VOTP. We find narratives, experiences, regrets & reminiscences - but messages? Maybe not, at least none so overtly declaimed, and certainly none so political, Lord preserve us, unless in their interpretation, but never in their intention."

I think you may be missing something about contemporary folk song. It ain't just Billy Bragg who breaks your rule. Why, Christy Moore, Leon Rosselson, Dick Gaughan and Roy Bailey do it all the time, to name but a few. I mean, is it folk? ;-)