Even though that exclusion is present in P2P (e.g. Wikipedia) it can be argued that it is not its cornerstone. It is non-rivalry rather than exclusion what lies at the foundation of what P2P is. Weber (2004, p.154) went further introducing the concept of "Anti-rival" goods
"Call it a network good, or antirival good (an akward, but nicely descriptive term). In simpler language, it means that the value of a piece of software to any user increases as more people use the software on their machines and particular settings. Compatibility on the standard sense of a network good is one of thee reasons why this is so."
Interestingly this implies that for any user, either contributor either free rider the mere use implies a contribution:
"The point is that open source software is not simply a nonrival good in the sense that it can tolerate free riding without reducing the stock of the good contributors. It is actually antirival in the sense that the system as a whole positively benefits from free riders."
There are of course limits to this, "This arguments hold only if there are a sufficient number of individuals who do not free ride (...)". Thereby in as much as there is a sufficient number of contributors free riding results in a positive effect.
The technological revolution has lowered the transaction costs of cooperation enabling new forms of production of the "Networked Information Economy" (B. 2006, p.106-116) such as P2P. Peer-to-peer is based on collaboration, on the capacity of its architecture to allow cooperation, to integrate and aggregate rather than exclude, it is inclusive rather than exclusive (B. 2006, p.99-116).
"Cooperation in peer-production processes is usually maintained by some combination of technical architecture, social norms, legal rules, and a technically backed hierarchy that is validated by social norms." (B.06,p.104).
Beyond the “Tragedy of the Commons”
Antirivalry has changed the problems that free-riding used to pose on commons enterprises. Free riding is usually considered as a drawback in collective action (Olson 1965 and Hardin 1968). Both Olson´s conjecture and Hardin's classic "Tragedy of the Commons" come to say that, paradoxically collective action hinders the consecution of collective aims. Olson (1968) conjecture establishes a negative relationship between group size and the ability to obtain collective goods due to free-riding. These however does not hold in the presence of antirivalry, contrary it happens the other way around, size becomes a positive factor in obtaining the collective good.
"The key concepts of the argument -user-driven innovation takes place in a parallel distributed setting, distinct forms and mechanisms of cooperative behaviour regulated by norms and governance structures, and the economic logic of `antirival´ goods that recast the `problem´ of free riding -are generic enough to suggest that software is not the only place where the open source process could flourish" (Weber 2004, p. 225)
Krishnan et al (2004) argue that free-riding does not undermine P2P processes, "free riding is sustainable in equilibrium and in some cases occurs as part of the socially optimal outcome". The architecture of P2P processes seems to be so that even free-riders might make a "passive" contribution. In "The Cathedraal and the Bazaar" E. S. Raymond recasts the term of free-riders into "outriders":
"Even at a higher level of design, it can be very valuable to have lots of co-developers random-walking through the design space near your product. Consider the way a puddle of water finds a drain, or better yet how ants find food: exploration essentially by diffusion, followed by exploitation mediated by a scalable communication mechanism. This works very well; as with Harry Hochheiser and me, one of your outriders may well find a huge win nearby that you were just a little too close-focused to see." (Raymond, 2000; p.15).
The condition for this to be so is that there must be a sufficient number of contributors (non-free riders). Benkler (2002, section III,A) lists reasons why there will always be a share of users that behave altruistically. Moreover, there might be non-altruistic reasons that might push users to contribute,
"(...) sharing could reduce congestion on these networks, which may increase an individual peer’s utility of using the network — providing a rationale for sharing even in the absence of `altruism.´”(Krishnan et al 2004, p.5).
P2P architecture might force users to "passively contribute" as in file-sharing services such as Bit Torrent which "(...) forces users to share the parts of files that they already own while they download the remaining bits" (Strumpf, K and Oberholzer-Gee, 2009, p.10).
In a setting where antirivalry is present free riders make a positive contribution, hence exclusion would have a negative role limiting access and reducing production. There would be then a positive relationship between group size and production of the common good. The set of social norms, legal rules and technical architecture that allows for cooperation in peer-production processes seems to result in a setting which can be characterized by antirivalry. In such a setting, where even free riders contribute to the production of the common good, it is inclusion rather than exclusion the most reasonable policy.