What is a suit of armor without a helmet? Answer: less effective. What is a helmet without a suit of armor? Answer: better than nothing. The same can be said of a shield and it improves one's armor class with or without armor, yet the helmet is nothing without a corresponding suit of armor and its absence results in a penalty. The inconsistencies of Dungeons & Dragons do not surprise me, but that doesn't mean they are not worth addressing.
If any historical argument needs to be made for assigning protective value to the helmet alone, one need only look to history. On the battlefields of ancient Greece, often the only protection a soldier had was a helmet, a shield, and possibly greaves. On the battlefields of the two World Wars (and many other wars of the modern era), a soldier had no protection but a helmet. If the helmet is insignificant alone, why is it the one piece of protection that is considered indispensable in warfare from the distant past to the present?
The 1st edition AD&D Dungeon Masters Guide provides no guidelines for use of a helmet without a corresponding suit of armor, but it does state that the lack of a helmet will attract more attacks to the head, which will have an armor class of 10. It's odd that no other missing piece of armor should likewise incur an armor class penalty. Basic/Expert D&D does not address the subject of helmets at all (as far as I can tell).
I propose that helmets should be treated as shields in that each should improve one's armor class by 1. The armor class rating for suits of armor should apply to the suits themselves. Armor class represents the character's protection in general. The exception to this is if hit location rules are in effect, in which case each hit location will have a separate armor class based on its protection.
Helmets, of course, restrict one's senses depending on the style of helmet, and they may have to be removed or tilted back or require the visor to be lifted to facilitate certain activities.