Hitting something going downhill at high speed is going to cause brain damage or worse whether you have a helmet or not. Crashing on descents is very, very bad news.
Styrofoam will only protect you in low speed collisions. Somebody was killed in the Giro d'Italia last year descending from hitting his head on a siderail. He was wearing a helmet, of course.
This is the problem with these kind of anecdotes: If somebody crashes wearing a helmet, and is OK, it's just assumed that the helmet saved him. If somebody is hurt and was not wearing a helmet, it's assumed that he would have been ok if he was. In reality, this is a completely fallacious assumption, and is not borne out by the data.
Helmets probably have a positive impact on low speed crashes, but it is small. Motorists would have significantly reduced fatalities if they wore motorcycle helmets (which are much more effective but impractical for bicycles), like race car drivers do, but they don't. Pedestrians have higher fatalities per kilometer than cyclists (and pedestrian fatalities are often due to brain damage), but they don't wear helmets. Why is this one activity singled out to wear a bulky safety yarmulke?