I don't think it's beneficial to know. It would be disheartning to hear (for eg) after 30 hours of labour that you're only 3cms!
With DD2 the midwife broke my waters (at my request) and said that I was 3cm, 50 min later she was born. And I think I was one of the lucky ones there. She was just sitting there waiting for the waters to break to make her exit.