A commons can be a sustainable structure, if information costs are low, and incentives prompt derivitave works.
As a football coach, I enjoy getting to matches early to observe warm-up techniques, and adapting them. If I drive past a training session I'll have to slow down and see what the coach is doing. Once I created atechnique session called "T-Ball Zulu 5000", and within a week I saw several other coaches doing the same thing. Needless to say, I've utlitised far more of their work than they have of mine.
The costs of claiming property rights to coaching methods, and of monitoring other coaches prohibits any private structure. I shan't claim it as a 'public good', but since coaching methods is largely about invention, once created it exists independent of the creator.
It therefore worries me to see academics arguing against file sharing or commons based governance of music and film. Academia, after all, is the purest form of a knowledge generating industry. And it's structure is commons based.
Every article is written under a highly constrained system of norms, from the necessity of an abstract or disclaimer, to the detailed presentation of the bibliography. Such norms are voluntarily complied with, as a cooperative measure. There is no "market in articles", the AER does not bid alongside Economica for an essay by Hayek, rather journals specialize, and authors need publications to enhance reputation.
Such a system is indeed underpinned by private property, as Professors form contractual agreements with institutions, or research centres. However the actual knowledge is communally owned - anyone is at liberty to use a published work as a means to creating a derivitive. Hicks didn't need to financially compensate Keynes for the groundwork for his IS-LM analysis, rather the norms of the system required him to have a citation. Then anyone reading Hicks, can also read Keynes.
So for Napster, see Jstor - we can get online, download a work, be inspired by it to formulate our own thinking, and produce a derivitave. For the creation of knowledge, and the innovation of ideas beware the pleas of the people trying to preserve their privilige: a commons needn't always end in tragedy, as shown by the most established form of all.
More of The Filter^ on creative commons...