It's also not just about the voice but the music that is being played as well.
Some people seem to think that a decent voice automatically makes a song good!
It's also about what is being expressed and how it is being expressed. What the words are about, what story, what poetic images do they create. You don't have to be a great singer to do that.
I'm quite disappointed at what is often called 'alternative' music because content wise (and even form wise) it isn't very alternative.
PiL were commenting on how music is constructed, Lydon was taking a swipe at writers, at the politics of the industry and the hypocrisy of some pop stars. It isn't a comfortable listen and it wasn't meant to be. 'Metal Box' is a complete artistic statement which has not been emulated often since.