And now for something completely different.

PURE SPECULATION

Some of the songs with nonsense refrains might be holdovers from a purely oral tradition.

Think of a ballad as a data-stream.

data - there was a lady lived in york
end of data marker - all alee and lonely
data - fell in love with her father's clerk
end of data marker - down by the greenwood sideo

The refrains mark the end of a data transmission.
Also gives the singer a chance to remember the next item of data for transmission.

Each verse-refrain combination is a complete packet that breaks down into four pieces.

This consistency of structure plus rhyme plus meter would help the singer remember and sing the song.

Russ (Permanent GUEST and serious NERD)