Some performers want to be "authentic" in the sense that they try to reproduce a typical, or at least imaginable, performance at the time, place, and sociological context where the music originated. If such a performance includes a piano, there will be no reasonable opposition.

It is also undisputed that creative adaptations, like Benjamin Britten's, are legitimate, and must be judged in their own right.

The interesting problem of this thread is about performers who declare themselves traditional, in the sense that they claim to take part in the development of the tradition. They have the right to use new instruments, but must be very careful about their cultural signals.

I agree with Howard that the major "revival" movements were not valid by that criterion, since they imposed new ideologies. Yesterday I leafed through the Cecil J. Sharp collection that Tom rightly described as "streamlined". Since it was printed in 1916, it also inevitably carries an ideological subtext: "In times of national need, the aristrocratic and bourgeois ladies must demonstrate their solidarity with the common people, on the condition that it does not smell." Which has further discredited the piano for performers who want to represent the common folks. (Note that this effect is by no means restricted to Britain; most European folk traditions suffered similar abuse.)

The guitar does not have that particular sociological problem, but, like all other instruments, it is sometimes used in an incongruous style. Many folk songs, even American ones, do not fit into the drawer "the times they are a-changin".