Context can be interesting in itself, whether the song can be sung without it or not. It all helps to space an evening out.
To pick up on one of the points above, John Kirkpatrick said recently about one of the songs he sings: if the words don't quite fit the music, change the music. I don't think he meant it as a general rule for songwriting though - ideally the two should flow as one. Perhaps the songwriters in the thread could comment on this: is it better to maintain a common musical accompaniment throughout a song (verse on verse) or does some variation actually help make a song stand out?