The way I see the argument going is like this.
Obviously (I expect) Were I to propose to whack Weep over the head with my "Clicky-Ba" he would prefer that I did so after he had put a helmet on. So in absolute terms, wearing a helmet is safer.
However, we are not looking at a simple enviroment. Does the wearing of a helmet make it more likly that cyclists will be knocked off (Perhaps because drivers see helmet wearers as somehow more experianced and so give them less margin) to such an extent that the benefits of helmet wearing are offset by the greater risk of accident?
The analogy with the building site is not so daft. I imagine that before hard hat wearing became more or less compulsary builders were extremely careful about not dropping small items that might cause injury, perhaps now "everybody wears a helmet" they may be less careful about this, if only by a small margin.
Of course there is also the traditional "Risk compensation" effect, Previously non-helmet wearing Cyclists might feel invincible wearing a helmet and so take grerater risks and therefore suffer more accidents
Unfortunatly these are actually very difficult effects to objectivly measure because there are so many additional factors (New cyclists, especially middle aged "Born again" ones are more likly to wear helmets but are also, due to inexperiance, much more likey to be involved in accidents, how do you account for this??)
There is the other argument that, under certain circumstances, a helmet might exacerbate what might otherwise have been a minor injury. I am sure that this does happen. I am sure it happens with seat belts too. but I am also sure that (Like with bad vaccene reactions) individual cases of disadvantage are rare and that the overall "Herd" benefit is still substantially positive (Again, like with vaccenes)
There is of course the other argument that cycle helmets do little to help in major accidents (such as being dragged under a bus/lorry) and ususally make little diference in minor ones. But in intermediate accidents they allow people who would otherwise have been killed outright to survive though seriously disabled. Is this actually a good thing? (Either for the individual or wider socioty)

(The above also applies to motor vehicle safety devices too...!)