From a scientific and engineering stand point, frontal crash test ratings are only comparable for vehicle within the same weight class (this fact is also clearly stated in both IIHS and NHTSA website). In other words, if a good frontal rating car that weigh 3300 Ibs crashes head-on with another good frontal rating car that weigh 4400 Ibs (like Tesla). The 3300 Ibs car frontal rating would drop from "good" to "poor" because the lighter car would decelerate much faster than the heavier car in the collision. (faster deceleration means more injury)While I'm not trying to dismiss the IIHS testing methodology or results, nor am I defending Tesla's response to them, I do want to say that the real world crash examples have indicated time and time and time again that Tesla built cars are extremely safe and many people have walked away from accidents they may not have otherwise. Whether or not this matters to you over the results of a "independent" test, is entirely up to you.
As for me, I can say without question that I have never once had a concern for my, and my families, safety while in my Model S.
Jeff
Another way to look at frontal crash rating is that the "good" ratings are valid ONLY if:
1. Your car get into a single-car accident. i.e. your car hits a tree or pole, no other cars are involved.
2. You car get into a multi-car accident, where the other car weigh about the same as your car, or it weigh less than your car.
If your car get into a multi-car accident, where the other car weigh more than your car. Then you can pretty much throw the frontal rating out of the window. It doesn't apply anymore. That's where the theoretical crash test results versus real world results argument comes in.
It is simple physics, a bigger/heavier car is always much safer than a smaller/lighter car in a crash. Despite Tesla having only "acceptable" in the small overlap frontal test (and frankly pathetic response by Tesla PR team), Tesla vehicles are still significantly safer than most cars in the market simple because of its weight advantage.