The purpose of IP law

I've always found Lessig's explanation of the history of intellectual property law to be quite persuasive: that copyrights were not invented to ensure that artists make as much money as they can off of their creations, but rather to provide enough of an incentive to create new ideas that the creative process does indeed continue. Fundamentally, copyrights are not for the benefit of the artist (or distributor, or whatever); rather, they are for the benefit of society. Providing limited ownership to creative products provides that incentive; however, copyright law shouldn't be extended past the point at which it increases the creative output of society.

(This is certainly very different than our typical capitalist approach to property rights, but I think Lessig is right that this was the original rational for IP law in the western legal tradition.)