I always use a bash hat when I go out on a bike; but that's my choice. An interesting point made to me once by a non cyclist was 'If there was a pill / vaccination that would reduce the chance of your getting a potentially crippling / fatal illness, would you take that pill / vaccination - ?' He meant of course for 'pill / vaccination', read 'bash hat'. Interesting and valid comparison.
Another comparison works on the principle that most head injuries occur in groups other than cyclists
It is a bit like having a pill that prevents a serious illness in adults... then prescribing it for children