The point of copyright is to have a long enough period to encourage people to make their own works.
I'm going to nit pick this, but don't take it personally because I think you get it, but the overwhelming majority don't. Whenever people say/write this I think they believe its the exclusivity itself that is the motivator for artists to create works. I totally reject this idea. Read this sampling of how musicians feel about online "piracy."
My take away from it are that the artists either don't care or are encouraged by it. To them, their art is an experience. It's either the experience of going to a show to witness a live performance, or like Jack Black's comment, holding the vinyl and seeing the album art in its bigger than life glory and reading all the extras in the album notes. No amount of copying is going to stop people from going to live shows, and no amount of copying can hurt real album sales. The recording industry got cheap and lazy and shot themselves on the foot when they reduced themselves to being the packagers of only 1s and 0s.
Its the old guys that are attempting to live off the residuals and the royalties of stuff they did 20, 30, 40 years ago that dislike it so much. I took Prince's comment as "Shit, if I can't make my % on sales, I am going to have to get off my ass and work again."
And now, to my point, I argue that it's "for limited Times" which serves as the motivator for artists to create. That copyright expires is the most important part in the copyright clause. If I write a book or two, win success roulette, and maybe get a movie deal which nets me enough money for me to be more than comfortable for the rest of my life, my children's lives, and most of my children's children's lives, why would me or my children work? Pure greed, more money? Greed doesn't make for good art.
I'm getting off the path now, but its the same story with healthcare in America. The people were tricked by the leeches to believe that the cause of excessively high healthcare costs (insurance) is the solution . The problem with copyright / being able to create new works is that copyright lasts too long.