That is a serious paradox in kiting. The risk of high speed impacts vs. the risk of high speed impacts. Usually on land, when you have risk of high speed impacts, well designed helmets are as substantial as feasible to address those impacts. This means multiple foam components, usually a thick polystyrene (EPS) or styrofoam foam layer that "self destroy" or crush on a single impact conveying impact protection through energy conversion and a substantial shell. Styrofoam works very well for this. There is a problem though.
EPS is for single impacts, it was thought water users would have more frequent impacts. This would mean discarding helmets after impact even lesser ones. For this and other reasons "multiple impact" foams have been developed for water activities for sometime. Many think EPS excels multiple impact foams in impact related energy management. At the same time, the water helmet market appears to be focused on multiple impact foam.
How does a helmet work? This is what they Snell Institute has to say about it.
"How do helmets work?
Helmets are normally comprised of four elements; a rigid outer shell, a crushable liner, chin straps or a retaining system and fit or comfort padding. The rigid outer shell when present adds a load-spreading capability, and prevents objects from penetrating the helmet. It's kind of like an additional skull. The liner, usually made of EPS (expanded polystyrene) or similar types of materials absorbs the energy of an impact by crushing. The chin strap when properly buckled and adjusted along with the fit padding helps the helmet remain in position during a crash.
Helmets work kind of like a brake or shock absorber. During a fall or crash a head is traveling at a certain speed. Since the head has weight, and is moving there is a certain amount of energy associated with the moving head. When the helmet along with the accompanying head impact an unyielding object; a rock, a wall, a curb or the ground the hard shell starts by taking the energy generated by the falling helmet (head) and spreads it over a larger portion of the helmet, specifically the internal foam liner. The foam liner then starts to crush and break which uses up a lot of the energy, keeping it from reaching the head inside. Depending on how fast the head is traveling, and how big, heavy and immovable the object is the faster the head slows down, and the more energy is present. In short everything slows down really quickly. A helmet will effectively reduce the speed of the head by breaking and crushing which reduces the amount of energy transferred to the brain. The whole process take only milliseconds to turn a potentially lethal blow into a survivable one."
Still more with focus on water helmets as opposed to EPS foam models at:http://www.kiteforum.com/viewtopic.php?f=131&t=2313506&p=202276#p202276
There are lots of good (read: bad) reasons for wearing a helmet in the posts on this forum. One in particular worth looking over is AJ's story:http://www.kiteforum.com/viewtopic.php?t=2312791
A helmet article from The KiteboarderHERE
A SBC Kiteboard article on helmetshttp://www.kiteforum.com/viewtopic.php?f=131&t=2303394
A fairly low speed but tragically fatal impact involving an advanced kiterhttp://www.kiteforum.com/viewtopic.php?f=131&t=2316234
Might as well throw mine in there, got me started on this ten years back.HERE
I'm all for wearing helmets while kiting.
But to get the discussion into perspective the standards approved helmets are for kayakers rolling over and hitting a rock, not kiters flying through the air at 50km/t.
So helmets reduce injury, but they are not appropriate for the chosen activity. Bicycle and motorcycle helmets self-destroy on impact, water helmets do not.