The case against patents can be summarized briefly: there is no empirical evidence that they serve to increase innovation and productivity, unless the latter is identified with the number of patents awarded – which, as evidence shows, has no correlation with measured productivity. This is at the root of the “patent puzzle”: in spite of the enormeous increase in the number of patents and in the strength of their legal protection we have neither seen a dramatic acceleration in the rate of technological progress nor a major increase in the levels of R&D expenditure – in addition to the discussion in this paper, see Lerner  and literature therein. As we shall see, there is strong evidence, instead, that patents have many negative consequences.
We can only conclude, at this point, that people who favor patents on “utilitarian” grounds are either ignorant or dishonest. They are much like the leftists in Sowell’s Vision of the Anointed: Self-Congratulation as a Basis for Social Policy, or those derided by Charles Murray in Losing Ground, or an analog of Isabel Paterson’s humanitarian with a guillotine. Or maybe they are just misanthropes or Luddites.
Update: the paper is now published in the Journal of Economic Perspectives.