Case in point: "Elementary" [1] is a Sherlock Holmes series where both Moriarty and Watson are women, Sherlock Holmes lives in NY and he doesn't get along with Inspector Lestrade.
If things never fall into public domain, doing something like this can be either very expensive (e.g. paying royalties) or plain impossible (e.g. the author simply doesn't allow it).
Copyright laws were meant to incentivize authors to create more, but no amount of incentives will make Sir Arthur Conan Doyle create more interesting stories, about Sherlock Holmes or otherwise.
We accept some limits on this right so that creative people can earn a living, but fundamentally, people have a right to the stories and images that shape their lives and the lives of people around them.
If the works of Shakespeare were still under copyright and still controlled by some distant descendants, and those people decided to disallow any new adaptations or printings of his work, it wouldn't just be unfortunate; it would be unjust. The works of Shakespeare now make up a part of our shared human experience, part of our understanding of the human condition, part of our lens for viewing the world. The were created by one person, but now they have been merged into the common culture it is unjust and undesirable for them to be controlled by any one person.
It highlights in numerous ways why public domain is essential for creative process. But it boils down to this: there is nothing new under the sun and the spirit of the copyright law is "to promote the progress of science and useful arts," as written in the constitution. The current abuse of copyright law causes the exact opposite.
As the article says, public domain breaths new life into forgotten works. It's culturally important to lift the access and publishing restrictions of old works that we learn from and reflect on where we came from.
Historical art is important to celebrate, rediscover, republish and enjoy without fear of copyright takedowns by faceless corporate stooges looking to line their pockets.
One possibly crackpot theory that just occurred to me is that in the _Futurama_ universe, copyright was extended so far as to be perpetual[1]
[1] http://www.cc.com/video-clips/ubbd92/futurama-nibbler-s-birt...
And of course you can get digital copies from sites like Project Gutenberg.