Helmets help a lot. Not only because they have a good chunk of AC, but also because the Perk system rewards you a lot of AC for wearing a full suit of armor, which includes a helmet. Plus, it's another item that can carry enchantments on it that you're missing out on if you aren't wearing it. Though, in all honesty, the game isn't hard enough where you'll need all that extra AC and enchantments, but not wearing a helmet greatly reduces the entire point of trees like the Light and Heavy armor trees. That said, a no helmet mod usually tweaks the location of the helmet to another slot, like the ring slot. Doing so would then make you have to choose between everything a helmet might bring, especially if perked for it, vs whatever a ring might give you. Pretty sure the helmet would win out in that fight, but it's something to think about I guess. I'm no modder, so I don't know if doing that would require the as of yet unreleased toolkit or not.