I speak from experience, here is a recent example from my own experience, I had a gig cancelled at Mansfield folk club on the pretext that the previous organiser had booked two concertina players within six weeks, never mind that they would happily book guitarists more regularly.
after much argument,I persuaded the new organiser to give me the booking at a later date [ nearly a year later],In fact I had a great gig at the club,and really enjoyed the club.
but it was an example of the idea that its ok to have x amount of guitarists in a given period but not concertina players.
Peter when he was on song was an exciting and exhilirating performerof his kind, he was an up front in your face performer, I have known other great performers like Roy Harris, who were great perfomers of a different kind ,.
which brings me on the other prejudice that i have witnessed against solo unaccompanied traditional singers