When I first became "involved " with British folk music, I was at Primary school , and listening and participating were compulsory !!So, apparently, was piano accompaniment, as most schools, both primary & secondary, had both a piano AND a music-teacher. One sat in front of the other, and a folk-song ensued !! This arrangement was entirely satisfactory given the circumstances, although in retrospect, some things did sound a little odd.However, as the piano existed principally for the purpose of facilitating music ( ie Piano ) lessons, then its use was not considered by us novices to be either detrimental, or,indeed, even controversial.HOWEVER---!! As we, alright then, I , grew older , one ( that"s better ! ) became aware of other ways of accompanying folk-music , and the other instruments that were suited to this role, whether "traditionally ", or not.By these one means, usually, the guitar,concertina,harp,accordion,recorder,fiddle, and maybe the organistrum (hurdy-gurdy) , although there were not that many " orgasmatrons " around at the time . Oh !---and not to forget the Portative Pipe-Organ, as played by Dolly Collins !! After that, it all became a matter of personal taste. I personally DETEST the sound of the piano in certain situations, and appreciate its subleties in others. For example, the sound of a good Shetland fiddler being accompanied by a pianist is, for me, UNBEARABLE !! I never understood why Dave Swarbrick did some work with piano accompaniment, but then I always preferred him on mandolin ( Stunning !! ) anyway. Whereas,some modern 'bands', especially those from Cape Breton, just wouldn"t sound the same WITHOUT a piano, albeit electric, and I LOVE the sound !! So, you see, it"s like beauty , in a way, in that it"s all in the ear of the listener. As to whether ANY instrument can be described as being "traditional", I really couldn"t say. With regard to folk-music, however, I WOULD tend to discount the piano,although it was "fashionable " in Victorian & Edwardian times,whilst coming down on the side of more portable stringed instruments such as the lyre and hammered dulcimer, which have existed for millennia, and are much more "Portative " !!!