Some people focus on the fact that long copyright terms help artists make more money from their work. But I don't see that as the real effect of long copyright terms. See this article on The Atlantic that shows the effect of long copyright terms. It's really profound what a small graph can tell you.
The effect of long copyright terms is a kind of artificial scarcity, I should call it artificial extinction. When an author or publisher is no longer interested in a book, yet it is still under copyright, it becomes very increasingly hard to find. This makes older works entirely unavailable for many. This is really bad for our culture, but it's good for the culture industry. Because the culture industry doesn't want to compete with the public domain. A healthy collection of public domain works which are de jure free will hurt the creation of new works because people can just tap into the public domain for content.
When you see pro-copyright people complaining about Creative Commons, they are actually channeling this idea. Creative Commons is an artifact of copyright itself, how could someone who is pro-copyright be against it?
The problem is Creative Commons is competition. You'll see some copyright crusaders promoting Britannica these days. Really? Why? I read and contribute to the free encyclopedia Wikipedia. I'm sure you've heard of it. I like most others have no need for Britannica, which contains a tiny fraction of the content available on Wikipedia. So really, why have they suddenly become Britannica fans? It's quite obvious because Britannica's business model is based on the typical artificial scarcity concept and it costs money (pre-Wikipedia it used to cost a lot of money). Attributes they like in their cultural works. Wikipedia has neither, it's actually a free culture project. Likewise, I don't see why I need to buy music from iTunes at 99 cents a track when I can download thousands of tracks from Jamendo for free. When you tell pro-copyright people this sort of thing, they get extremely uncomfortable. Because they have no easy way to react. Suddenly their usual rhetoric feels cheap.
Public domain is a threat for the same reason. And how do you kill public domain? Extend copyright durations of course. The public domain is essentially frozen in time, just the way they like it. But is that really good for society? Hell no.
And this sort of thing encourages piracy. Now if someone wants this content, they can't even pay for it. Suddenly file sharing looks less like "evil pirate activity" and more like the most effective systems for ensuring the preservation of human knowledge and culture.