Jimbo, I'm not an engineer but my understanding is the car seems to disintegrate in order to dissipate the energy of the impact through the entire vehicle instead of concentrating the impact.
In the early years of NASCAR and Indy cars it was assumed that a solid or rigid car was safer, Later, it was found that if the car dissipated the energy of the impact the effect on the driver was much less.
In Indy cars the tub the driver is cocooned in is the only "rigid" portion of the car. Same as the cage in NASCAR.
The way Wheldon's car impacted the catch fence (flush with the top of the cockpit) there was very little chance the outcome was going to be good.
My own opinion, the Indy cars need to stay off the tracks with progressive banking. It's basis of design was to provide more racing grooves for stock cars.
After this and the Texas debacle, I hope Indy car takes a hard look at the type of ovals they are racing on. If they are to race ovals and I like really Indy oval racing, "flat" tracks like Indy, Pocono, New Hampshire and Milwaukee should be where they go. Not Texas, Homestead, LVMS,etc.