The need for a core seems to go along with @mako 's research, although he seems to think it leads to the core benefiting themselves from their positions to the detriment of the project's mission, which can definitely happen. Is it inevitable?
Scuttlebutt may be an instructive counter-example. They welcome new people all the time, and one member of the core is also an active and visible mentor of new developers. And they have spawned several new projects.