For me, the song is paramount, the perfect melding of words and tune. The band needs to play behind the singer(s). But when there is to be an instrumental break, this is the place for the band members to strut their stuff -- grace notes, doublings, glissandos, countermelodies, et al., go crazy! But then drop back for the singer. This kind of blend allows the listeners to listen to the song and also gain an appreciation for the instrumentalists.

For further reading, see the notes on accomaniment in "The Singing Island," MacColl-Seeger, and "The Penguin Book of English Folk Songs," Vaughan-Williams-Lloyd.