I agree with Harvey about the way a syllable added or missed can change the whole feeling of a line away from the way you wan it to be, and not just because if scansion. The little details that people can so easily ignore really matter.

The thing is though, I know that the version that gets put on paper or on tape isn't all that likely to be the definitive final one, and there has to be room for the song to adjust itself.

Where a singer has held onto a song, and has sung it through that stage, till a version of it has been reached that really feels right, before it ever gets pinned down in print or on a record, that is a different thing.

That Richard Thompson song - I assume that he was intentionally avoiding the rhyme table and able because it felt a bit facile. So the line he puts in gets seen as contrived. Can't win.