The crumple zones on cars already act like cushions. However, it might be possible to reduce the thickness of some of the steel parts if they were filled with foam. The manufacturers probably wouldn't want to do this because it is cheaper to just use more steel. Foam would be nice though. It would lighten the car, deaden noise and improve thermal insulation. It might be bad in a fire though, as it might produce toxic fumes.

I think a more salient reason that foam inside the fenders would be bad is that initially, the foam would seal to the metal, but after a couple of thousand temperature cycles (a year or two) there would be a gap between the two that would be perfect for retention of moisture and it would rust-out the car. Also, moisture is bad for urethane-based foams like 'great-stuff'.

The insides of the fiberglass panels of my Pulse were foamed with urethane, and they're yucky now with a water gap and the foam pieces are mostly loose. I just scrape it out. I don't know if I'll refill them later--havn't decided. If you don't know what a "Pulse" is,

www.autocycles.org

I assumed there'd be spaces between the foam and steel to avoid moisture buildup and rusting. Perhaps this would even add enough protection to make some parts that are currently steel out of (recycable) plastic.

Extruded foam isn't bothered by moisture, and I'd spec'd a fire-retardant to be mixed into the foam--it might even help put out a fire!