Ok, lets go to the data on this one....
“The most commonly occurring bony injuries were fracture of the ribs (72.3%), skull (55.1%), facial bones (49.4%).” The data was drawn from autopsy reports and suggests that head injuries, if not always the fatal mechanism, are nonetheless common.
https://www.researchgate.net/publication/10775101_Analysis_of_injuries_among_pilots_involved_in_fatal_general_aviation_airplane_accidents
A helmet won't help you in all situations (like you said from landing in water), but think of it this way, if you land in water and take a knock to the head, do your chance of survival increase with a helmet preventing a knock-out and drowning or does it give you a fighting chance to get out of the plane while holding your breath..... Substitute for fire on the ground.
We like to think (and certainly advertise) that our low speeds are what keeps us safe, but at 30kts hitting an object, do you know what that would feel like? Tim Howes did a great interview on helmets a while ago from an actual accident that occurred at STOL speed where the pilot was thrown from his seat (seatbelt failed as well) and ended up with his head between the fuselage and the passenger seat where it hit a pretty small tree. If he had been wearing a helmet, he would have walked away (in theory).
Again, I am not saying a helmet will save you everytime, but consider the risk of the ground not being perfect, having equipment failure of a tyre or a suspension component, or a steering component failure and if you hit something in your plane doing 30kts as a stall speed (which most of us are around 45kts), you have bugger all soft spots to bounce around inside your cockpit with.
If you are hitting something hard enough to break your neck due to the additional weight of a helmet, I think you have bigger problems than the helmet itself....
But this is a good conversation to better understand the reluctance of people to take a simple protective measure. Its probably not for everyone, but for those who see the sense in it, I wonder what is holding them back....