If you strike the cells, they can be short circuited, leading to heating. Lithium cells can experience thermal runaway [http://en.wikipedia.org/wiki/Thermal_runaway#Batteries] when they're failing, heating adjacent cells causing them to fail. This is why cells are compartmentalized. Think of them like properly engineering bulkheads on a ship. If certain compartments fail, others are safely isolated.

The problem is, as long as drivers demand range with electric vehicles, and energy storage is much less than liquid petroleum, the bottom of the vehicle is the only place that large, heavy pack arrangement can go. You make the tradeoff: How much armoring needs to take place to avoid the most common damage/failure scenarios.

I have no doubt Tesla is learning quickly from these events, and is plaining alternate or improved methods to armor the pack from road debris and damage.