Do you assume the 7.5 limit of armor weight is true of bows along with thrown and crossbow?
I'd assume that the weight limit (which after my crossbow test is now speculated at 7) is universal across the board, but I don't have any hard evidence. I might test it when I get the time; it's easy enough to test yourself, though.
Most of the times a helmet is never needed, if anyone gets a headshot, oder hits you in the face with his weapon you will be dead anyways, a headarmor of ~15-20 wont save you.
If you're at an odd angle of attack or they're at the end of their attack animation, 20 armor can save your life. Another thing is that you're less susceptible to low-powered hits that might contribute to your demise; peasant weapons, rocks, throwing knives, etc. I agree that most of the time it won't help you if the enemy has a powerful enough weapon, though.