I'm not sure that's a fair comparison. The Tesla hit a steel rod at a high rate of speed, and the rod punctured the battery pack with something like 26 tons of force. The battery pack was designed to contain the fire, and would have if the firemen hadn't tried to puncture through to get to the fire. If the battery is properly protected, it's no more dangerous than hitting a metal rod in the road would be to any other vehicle.
Same thing can happen in a Volt, however, the chances of it happening in a Fusion, Camry, Cmax or Prius is very low. It would go through the fuel tank before hitting the HVB. There is a risk involved in any of these, and that was proved with the Tesla. What are the odds of hitting something that can pierce the battery pack? Pretty high, yet it happened. Then the Volt pack that got damaged in the Volt during crash tests. Didn't catch fire right away, but after sitting for a time. A slight flaw that GM figured out how to correct.
The point being that the HVB needs to be well protected because of the high voltages present and the fact that a breach to the medium can cause an immediate fire, so putting cells in the body panels, would not be a very good idea.