So TRPGs are trying to emulate TV shows and plays? I'm not so sure, since in a TRPG, a helmet (usually) doesn't conceal someone's identity.
It's fair for rules to assume that a helm is included in a suit of armor, but I was more interested in the "appropriate" part - does full plate assume a greathelm? Does leather armor assume a skullcap, or no helm? If so, lightly-armored characters have a much greater chance of disfigurement than heavily-armored characters do. But they can probably see and hear much better.
Re: other posts...
I've never heard someone say, "I use my action to ready my shield." So I suspect that if helmet rules were more common, there wouldn't be requirements to don helmets, either. The armored fighter could clank along quietly in the dark, listening and watching without her lid, and throw it on once combat begins, right? (Personally, I'd require actions for these things.) So if a helmet doesn't require the use of a hand like a shield does, what's the cost of using a helmet? The value of seeing and hearing in combat shouldn't be minimized, here.
Sight - combatants need to track enemies, see their weapons and stances, see the battlefield, and see their footing. It's easier to get "flanked" when you have limited vision, and worst case, an enemy could use hiding/concealment rules against you if your visor is exceptionally thick, or say, covered in mud.
Hearing - key for communication with comrades, but also for preventing flanks, and gathering info about what you can't see. Ear/head safety contributes to balance.
A helmet might just be "another piece of armor," but it's not protecting just another body part. You could say that wearing a helmet is an important choice for a player, even if attacks don't/can't target the head. A helm, given the head's importance, could, for example, provide a defensive bump similar to that of some shields, with a cost of losing some ability to locate enemies, to call for healing, and/or to execute some maneuvers (if the helm is bulky).