Dan
This has to do with safety and insurrance but also if the helmet is your own or property of the group/organisation your in.
And with some people in the battle line, i prefer a thicker helmet.
Made from modern single plate i personally would no go under 2mm, even with an arming cap.
I played, fight/training, a little bit with somebody from Poland a few years back.
He was then a professional stuntman.
I was wearing an early dpka model ridge helm with an arming cap under it, and felt the headshots very well.
But no headaches or other injuries, just plain fun.
My opponent was dressed in 14th century full armour, brigandine and the rest.
Some people on this forum might remember this.
Cesar.
Correct, authentic plates are stronger.
But not everybody will or can pay the price for a real replica.
Let alone being hit on such a helmet
See this topic
http://www.romanarmy.com/rat/viewtopic.php?t=21763