So am I right in thinking that the argument from a number of posters is not whether helmet's should be worn in a Chipmunk but that, for safety and security, they should be worn in all types of light aircraft including helicopters?
Not from me. Flying is inherently risky and we each need to find a level of risk/reward with which we're happy. That can include wearing a helmet or not wearing a helmet. I could entirely understand why someone might not want to wear a helmet in a tiny bubble canopy on a sweltering hot day, for example.
What I don't like is people either kidding themselves that flying is much safer than it is (one of my pet hates is to hear the 'safer than driving to the airport' trope from GA pilots and particularly instructors) or to hear other people being put down for taking what they see as sensible precautions to reduce the risks.