But from the (very theoretical) standpoint of doing this with a significant portion of the world's trash, the advantage is that the energy is already just sort of sitting there for us to use, rather than having to burn oil or whatever to heat something to a couple thousand degrees.

This is why we need to build a geodesic dome over the caldera with a chimney that vents into space.

Well the heat is free anyway, transporting it up to the top of the volcano isn't :)