An Open Peering Policy is a reflection of the value of Internet Peering - the value of the local optimization of routing.
Brokaw Price (Yahoo!) calls this "Pulse Peering" ; if you have a pulse, we will peer with you.
In either case, we tend to see Open Peering Policies from the Large Scale Network Savvy Content Providers and many of the CDNs and Tier 2 ISPs who seek to reduce transit costs, improve performance, gain some marketing benefits, etc.
It is interesting to note that in 2008, Brokaw Price highighted the fact that they had an OC-48 global backbone supporting thousands of peering sessions, and the operational overhead of peering was not an issue. For a content company, they have impressive network infrastructure to support the directive of a high quality end-user experience. Google, Microsoft, Electronic Arts, and the other Large Scale Network Savvy Content Providers have embraced an Open Peering Policy for the same reason - end-user experience.
Peering Policies can be described in these these rough categories:
Definition: An Open Peering Policy is an articulation of an inclination to peer with anyone.
Definition: A Selective Peering Policy is an articulation of an inclination to peer, but with some conditions.
Definition: A Restrictive Peering Policy is an articulation of an inclination not to peer with any more entities.
Definition: A No-Peering Policy is an articulation of an inclination not to peer at all.