Copyright law was always a compromise between the advantages to the public of easily sharing information and the importance of ensuring creativity is subidised. I'd say the reason public domain exists is to hand the information back to the public so people can reprint and share as much of it as they like and benefit from the information. Long after the author has already been rewarded.
I just finished Free Culture. Lessig's definitely got a convincing argument. The problem isn't copyright per se, but the fact that our free uses are not clearly defined and the language of copyright law was written for a non-digital world. The way lawyers are using antiquated terminology to foist more and more DRM and restrictions onto paying customers is criminal.
What I didn't agree with is Lessig's assertion that long periods of copyright will inherently harm culture or creativity. So let me play the devils advocate: Why should songs or written works pass into the public domain at all? Just so Barnes and Noble can make pure profit off the sale of paperback classics?
What is so special about public domain works that couldn't be handled by the ebb and flow of the free market without the creator giving up his or her ownership? There is less demand for an old book so it must be sold for less money which would amount to pretty much the same thing we have now... cheaper books. Same with older recordings. They regularly get sold for next to nothing even though they are still under copyright And if a movie or commercial makes an old song hot again why shouldn't the artists profit? It's still their creation after all and somebody will profit so why should the artist be cut out? In spite of the issues with copyright system abuse I see no reason why the author or his or her estate should lose ownership of those works.