I've heard the two main prevailing thoughts of helmet manufacturers, for what a hard plastic shell does as opposed to just a foam helmet:
1. distributes force over the entire head, vs just impact point -- foam would just deform in that region without the shell.
2. keeps foam intact over multiple impacts.

I've used the Petzl Elios for this reason, but am going to replace it given that its near the end of its life. I'm looking at the Petzl Sirocco.

Petzl claims a safe mountaineering helmet due to the foam remaining intact over multiple impacts, but i don't see how this helmet addresses point #1.

Anyone have insight into the safety of the Sirocco vs hard shell?
http://www.petzl.com/us/outdoor/vertica ... ts/sirocco

Depending on the density of EPP used, these helmets can have very similar hardness to a traditional shell. This is measured by pressing on the surface with a pointed steel bar and seeing if it makes a dent.

The applicable safety standard is EN 12492. The penetration test included therein is this: a conical (60 degree tip) 3kg weight dropped from half a meter must not make contact with your head. (A good rule of thumb is, don't get hit by falling rocks, whether you're wearing a helmet or not.)

Here's a good summary: http://www.petzl.com/EPI/v2/epi-en/normes/norCasGb.swf