My feeling is that styrofoam is a pretty poor material for helmets. In my experience (about 6 broken helmets) they don't crush, they crack. While yes, this does dissipate some of the force, they are by no means living up to the claims made in the manual that they will crush down to 1cm thickness. A cracked helmet is a helmet that has failed imo.

The real reason they use styrofoam is that it's dirt cheap to manufacture.