I think when an enemy can do >80 damage in a non-critical hit (web and hardwood golems can do up to 94 damage on a hit, ultra golems can do up to 74), critical hit protection is required. At least for me it is. If you want to move critical hit protection from helms onto rings or some other piece of equipment from ioun stones, that's fine, but those items need to be accessible in Ch 3.
Personally, I'm fine with it being the way it was in early bg2 (all head slot items). I think critto has already made it that way, and that is why I am in search for that new file (because I am too lazy to edit things myself when someone already has).
My reply above was to simply remind that neither ioun stone, helmet, nor armor alone could be expected to realistically protect from criticals. A person in full plate armor (covered from head to toe with armor) and magically protected from criticals as per an enchantment on said armor - now that makes sense. That's probably more trouble than it is worth to code. So, making the game rules like they were in the original BG2 is fine by me; I am in total agreement with bulian in that I want crit protection. In general (not always), I prefer changes to the rule set to make it more like DnD or leave it like the original BG2.