Why on earth would so many old songs and ballads have choruses and refrain lines if it was not normal for people to join in?

You can find a few hundred examples of songs where the refrain is an essential part of the performance if you go to

Kist o Riches

and put "waulking" in the search box.

The content of waulking songs is hugely varied, from epic laments to topical satires. I would bet that a lot of the songs that Jim only knows from Irish tradition as reverentially performed solo spots started out as work songs like that.

Waulking songs are sometimes performed solo in Scotland, but when they are it's a degradation of the tradition. At least here, everybody can tell when the singer has gone off on an ego trip in Mod-competition style.