Although helmets may be safer in some extreme situation, such as kiting upwind of hard objects, kiting in squally on shore winds etc....
However, I see a few potential risks in wearing a helmet:
1. At a crash they increase the risk of a whiplash.
2. Because of their considerable weight they tend to move on the head.
3. They make it difficult to dive into a surf or shore break
4. They are bulky, so lines can hooked onto them in a messy wipe out (I would not go into details what happens then

)
It is like with vests, they are personal preferences, they have some advantages and some risks. Applying common sense, and read (and respect) the conditions highly reduce the need for them.