P2 is now a product, thus it’s both backend and frontend. A lot of features require tight integration to work. As of now, plugin+theme is the simplest way to provide this integrated experience, if this changes in the future we’ll review. Buddypress is a social network and discussion forum, while P2 is a collaboration platform. The goals are very different: one is open ended and built for socialization, while the other assumes socialization and it’s built to make things together.