I basically agree with Glen’s recent post, Wi-Fi, IP, Movie Theaters, and the Problem of Excessive Enforcement. I share his general approach to copyrights and patents: Their justifications depend on their social utility. And I loved his theater example! Good pedagogy, that.
A quibble: I would caution against the assumption implicit in Glen’s statement, "[I]t can sometimes be desirable to have imperfectly enforced property rights." Applied to copyrights or patents, that statement would apparently presume that property rights exist.
As a matter of practice, I grant it plausible and often useful to call copyrights and patents "property." The term requires caveats, however. I've argued that copyrights and patents strongly resemble welfare rights. Glen adds another caveat (if I may paraphrase): "Intellectual properties might deserve lighter enforcement than tangible properties do."
As a matter of theory, I would have anyone who takes up Glen’s topic keep in mind, and sometimes emphasize, that copyrights and patents do not qualify as natural rights. Those rights do not appear to have ever existed absent statute, and in contrast to tangible property rights look unlikely to arise as unplanned orders. Consent-rich societies can surely find other ways to generate expressions and inventions.