Why not PURLs or URNs?

PIDs are as said unique and persistent identifiers of objects that are made available by proper repositories. For many resources there are additional characteristics such as multiple copies for preservation reasons, a string (such as MD5) that can be used to check authenticity, simple metadata for citation purposes, a reference to the access permission record etc. A proper system should offer such information immediately when resolving a PID. PURLs can't offer functionality, for URNs we do not know about well-proven and robust resolver, although the big libraries agreed on using URNs for their publications.