That's still assuming that they help safety - "begging the question" as it's called. Of course, few of us want to injure our brains, hey some people's ar virtually unused after all, but the first question is whether, on balance, helmets help.
Without evidence, a "common sense" view is that helmets will protect the head. However, the Australian experience suggests on average they make little difference, with any good counteracted by harm in other ways.
So wanting to protect one's brain is likely a shared goal, but seeing as at appears that helmets on balance, don't help, what are we to do.
It maybe that they have more benefit in some scenarios than others, and so a judgement could be taken - but even for "more risky" cycling we'd not know if they caused more harm than good in that type of cycling (neck injuries, rotation, bigger head = bigger target etc).
Even if they did help on balance - and it doesn't appear so - there'd still be the question on whether cycling is risky enough to bother - compared to, say, climbing stairs, or going out drinking beer.
EDIT - to put the "common sense" view in context, there are a (small) number of recorded fatalities of children strangled by their cycle helmet staps whilst climbing trees and the like. That's a solid example of a disbenefit of helmets for an activity which you'd think they'd be beneficial for. There may still be net benefit in this scenario - the point was to illustrate the unexpected additional risks of helmets in what might seem a clear cut situation