I have to admit I don't really understand this theorem fully. As far as I understand it says that the work done on a volume of charge is equal to the change in the field energy inside the volume plus the energy density leaving the boundary. I guess that makes sense but then I did a calculating with an infinite coaxial cable where a current runs down the outer cylinder and comes back along the other. In this case there is a magnetic and electric field between the two cylinders. So you find an expression for the poynting vector which is nonzero which must mean that for every volume there is energy leaving it. I don't really understand this since the situation is completely symmetric in time since the currents are stationary. That doesn't look like energy is leaving any volume at any point.