Okay, cards on the table, I do not in most cases, wear a helmet when cycling. I say most cases, as the are occassions - more on that in a mo.
..............It reduces your chances of head injury quite significantly. If you don't accept that & want the freedom to sustain brain damage & facial injuries you are deluded.
Nigel, I have no doubt that if I were to find myself in some scenario where I crashed or were hit by a vehicle AND my head were to be in contact/colision with something, then a helmet would up to a certain point, reduce or possibly negate head injury.
But what are the odds/chances of being in such a scenario? I note in that very long report you linked to by the IEA, that right at the beginning of the report it said -
"
Bicycle crashes and falls are rare, but they can cause a range of injuries from minor to permanent disability or fatality. "
Further, whilst the study evidences all sorts of statistics about cyclist injuries and so forth, there is nowhere that I saw (happy to be corrected) that gave any detail about the accident scenario, e.g. age of cyclist, cycling experience level, cyling alone or in a group, type of cycling, (ie. on-road, off-road, minor/major road, in the dark (with/without lights), broad daylight) and so on ad nauseam.
As someone close to my 3 score years and 10, with many tens of thousands of cycling miles behind me, both here in the UK and in Europe, I make a judgement (aka 'risk assessment') on every ride as to the possible dangers I might face and take mitigating action where necessary. Said action my include wearing a helmet such as cycling in a peleton in my local group, or mountain biking or in icy conditions.
So far that method has worked for me and whilst I do come off my bike now and again, it is the hips and hands that have so far mainly suffered.
My point is that, yes, for certain scenarios, the helmet will be of benefit, but for my style of cycling, the risks as I see them are very low, so no helmet wearing for me (most times).
Most of the downsides to wearing a helmet have already been pointed out already, but to add fuel to my favourite reason it is that the principle put out by Cyling UK in the 'Safety in Numbers' is a good one(the more cyclists on the road the more aware other road users are to their presence etc) and in support of this I do not choose to portray casual cycling as a dangerous undertaking by it being necessary to wear a helmet.
You have your views, and I have mine and I suspect we will maintain that difference - happy cycling, and keep safe.