Happy New Year to you too Brian.
I think you are right that instrumentation doesn't inevitably detract from the storytelling aspect of the ballad, but on the other hand, it certainly can and quite often does.
I once attended a lecture given by Tony Rose at C# House on the subject where he argued that (to paraphrase - didn't record it unfortunately) ballads always needed accompaniment whereas it wasn't necessary for (what he called) 'patter songs' - have spent the last thirty years working that one out - still haven't. I think he, Martin Carthy, Nic Jones, and other performers of that ilk have ruined more songs than they've enhanced with heavy-handed or unnecessary (albeit extremely skilful) accompaniment.
Peggy Seeger, who is, in my opinion, one of the best accompanists on the scene, once described the role of accompaniment as similar to that of a stage set, an unobtrusive addition against which the text can be presented - sometimes desirable, even necessary, but on other occasions totally surplus to requirements. A matrix in which to present your song, should you feel it necessary.
I don't believe MacColl's technique applies in the same way, as the voice is essential to the song where accompaniment, at best, can only ever be a desirable addition. Even the vocal techniques I described can (and have been on occasion) destructive if badly applied.
My opinion anyway.
Jim Carroll