And unfortunately in IWD 2 there aren't very many magical helmets. Definitely not as many as there are in either BG2 or IWD1. It has been a while but IIRC, some of my first party I had finish IWD2 ended up without a magical helmet. Whereas in IWD1 every non-mage and even my gnome fighter/thief had one, and in BG2 I was constantly selling off extras. As RTP said they are "mostly" cosmetic in IWD2.
According to the manual, helmets protect you from critical hits in IWD2 as well. Quote from the manual page 131: Helmets: Helmets, made of reinforced leather or metal, cover most of the head, save the face and neck.
Helmets protect against critical hits.
Yeah, d&d 3.0 (and 3.5) have different rules about averting critical hits, cause what's the point in having keen weapon (and/or improved critical feat; doesn't stack in 3.5ed) if it can be averted by a simple helmet.
There is some equipment that averts criticals, but it's insanely expensive (in the "pen and paper" game rules, I don't know if there's any in IW2)
I leave the comments on when I play BG2 IWD 1 & 2. I have seen the "Critical hit averted by helmet" comment in both BG2 and in IWD1, but I do not recall ever seeing it in several plays of IWD2 though. So Crucis is probably right about it being a hold over.