"Less prevalent" seems unlikely, from the large number of accidents I'm aware of among myself and peers, maybe 25% have involved motor vehicles? Sure, they tend to be the more serious ones, but certainly if you do any sort of regular off-road recreational riding (esp. MTB) you're almost certainly going to come off quite a few times with no other vehicles involved. But I don't know how well my own observations extrapolate to the population as a whole - I suspect a large percentage of bicycle falls never get reported/recorded anywhere anyway.
Helmets are required around construction sites for health and safety. That's not an argument to require helmets for pedestrians. Any particular activity can be particularly dangerous, and you can mandate safety gear for that activity, but that has absolutely no bearing on stuff outside of that activity.
And I agree that MHL laws should be more flexible, and not just a blanket "you must wear a helmet at all times while on a bicycle" statement, which is what it is in Australia.
