I'm with Shimrod - up to a point - that there's a difference between a chorus and a refrain, and I appreciate Howard's comment about the old traditional singers showing each other 'proper respect'.

But I'd like to propose a further distinction within the category of refrains. It seems quite reasonable to me that an audience should join in (sympathetically, taking their lead from the performer's interpretation of pace, rhythm and dynamic) with what I'd call Third Person refrain lines, as in The Cruel Mother or Riddles etc. or Two Sisters.

However, I think there's probably a case for NOT joining in on refrains in such ballads as Son David or Lord Randal, where the refrain lines belong to the characters in the narrative -- and so are an important element in the whole interpretation of the narrative, according to the understanding of the singer.