I read an interesting paper on whether P2P is appropriate for an application or not. Although the paper is based on file sharing work, it seems to suggest (see chart on page 8, and conclusions at the end) that P2P may not be appropriate for global Internet telephony kind of applications where participants do not trust each other and the resources are of low relevance (since individual phone number/user identifier in p2p-sip are not interesting to many peers; unlike popular files). The reason being, too much overhead due to distrust, and over due to non-natural p2p network evolution if the resources are not popular.
However, if the trust problem can be solved in p2p-sip, I believe that p2p-sip will be useful because it can provide serverless VoIP at different scale -- small organizations, to global Internet. The current skype model may fit as an appropriate application for p2p because the protocol is closed, which makes the different clients to trust each other to some extent (i.e., the application is not malicious).