Also straying away from balladry for a moment. American styles (blues/gospel/jazz/rock) frequently use instrumental bits within the verse, which is different than instrumental breaks or short instrumental 'fills' between verses. The instrument plays the response part of a Call and Response pattern…

If I'd a known that my captain was blind, bud dah duh dot dah,
If I'd a known that my captain was blind, bud dahda duh dumb

… taking the place of the second voice. This style may be an exception to the Songs for Singing/Tunes for Dancing rule, or maybe not since the instrument is playing a vocal part.
- Phil