We should also remember that not only does the EPS absorb energy by compressing in bulk, but also by cracking. Cracks might not be evident upon inspection because the place where the crack is may not have separated enough to allow you to see it.
A cracked helmet experiencing another impact may have vastly decreased ability to absorb energy, even if the EPS foam is not already compressed much. The crack may prevent force from being transferred through the material, allowing the helmet to deform into your head more and transfer more energy to your head, which is not what you want.
I've always liked the saying "A 20 dollar helmet for a 20 dollar head"