Ah - I'm seeing helmet-use as a tactical decision, not action. Also, the thread is partially about the rules of the gear, so why do f20, PF2, and BRP treat shields differently...and do they have helmet rules as well, or are helmets just specialized clothing?
I'm using "f20" in the sense of all fantasy d20 games - all the d&ds, 13th Age, the Pathfinders, osr, etc. What I mean is that attack and defence in those games are resolved in a single roll, and that's why AC is used so that defence is subsumed within the attack roll. In BRP games attack and defence are two separate rolls - attacker makes the attack roll, defender decides if and how they will defend, then rolls their defence. In BRP's case armour absorbs damage, it typically does not influence whether the defender is hit or not. That is the essential difference between f20 games and BRP when it comes to combat, and AFIAK
most rpgs treat armour as a damage sponge, unlike the f20 games where armour changes the to-hit chance.
This means that shields (and any weapon) can be used actively in a parry action to defend against an attack. In Mythras you can use the shield as a passive block while at the same time making an active parry with your main weapon, assuming a 1h weapon + shield combination.
Yes, BRP games have helmets, and cuirasses, greaves, vambraces and so on, depending on setting. Not all BRP games use hit locations but when they do, armour and hp are broken up across different locations (seven for a human), so you can be armoured differently in different places, and each item of armour can be of a different quality and strength. You can also be wounded in different locations, and a serious wound in the leg has different consequences than a serious wound in the head, for example. It is perfectly possible to have a very protective metal helmet, a linen cuirass and cloth limb protection as a mixed armour set. In some campaigns you might want to enable PCs scavenging materials like chitin from tough creatures to make their own armour pieces, or take crafted armour from defeated foes. This works well in sword & sorcery settings, or post-apocalyptic ones where materials are scarce.
So there are plenty of options, decisions and tradeoffs you might want to make with armour (including helmets), but all that comes
before a combat and not
during one. With a shield there are also decisions you need to make before you get into a combat, beginning with what weapons you are trained to use, what is available to you and what you are fighting against. For example you might need to decide whether your character uses a large 2h weapon, or whether they use a sword and shield and so on. When it comes to being in a combat there are then tactical decisions to make when you are using a shield - (using Mythras) are you using a passive block to defend and conserve action points by not attempting an active parry? do you attempt a parry in the hope of getting a special effect and trying to trip or disarm your opponent? do you attack with your spear or do you try an attack with your shield with an idea to Bash your opponent off a bridge because the shield gives you an advantage when bashing and knocking back? These are the kind of tactical decisions you don't get to make with a helmet.
In an urban setting you might not be able to carry certain weapons around, if you consider the reaction you might get today if you walked into a bank carrying a riot shield and a motorcycle helmet... it's unlikely that a town would tolerate large weapons and shields, but in a certain setting it might be permissible to walk around with a rapier and dagger and/or buckler because those are personal defence weapons and not military weapons.
By this idea, shields aren't important because they're essential for defense; it's because they interfere with one's offensive capabilities (weapon wielding). Of course, assuming that seeing and hearing are less important for offense, and that fantasy characters can't wield a shield and sword in the same hand, or wield a weapon two-handed while wearing a shield.
A shield and weapon combination makes offense easier because it protects you from attack as you close with an opponent. This is why a comparatively unimpressive weapon (the Roman gladius) was so deadly, because it was part of a weapon system used with a large shield (scutum), and in a formation. The shield means you can get close to use the gladius, without the shield you have less offensive ability.
A more typical weapon combination is spear and shield, which both protects you and allows you to strike at a long reach, and defend yourself against attackers at range and close up.
Your table of helmets and shields is probably fine for an f20 game.
