The way I see it helmets are part of some types of armor. You could make them a separate entry, but then you had to reduce the AC bonus of those types of armor. Otherwise, you're supposed to wear a helm, and it isn't mentioned. Of course, you can exchange that helm for a special one (magical helmet).
I really don't like the rule of helmets making immune to crits. That would make crits totally useless (and sneak attack, too). Rogues already avoid undead, oozes, constructs and barbarians, cause they cannot sneak attack them, should they now have to avoid every humanoid, monstrous humanoid, outsider, and whatever is able to wear a helm, too? Nope, if you want to wear something that helps you against crits and sneak attacks, go buy an armor of fortification.
Also, crits and sneak attacks aren't only hits on the head...
I could see helmets giving a bonus to concentration (like in NWN), and also a penalty to listen checks. But I really think helmets should remain part of the armor types, not some different entry.