The 500 man....
There is little doubt that a properly (professionally) fitted helmet has good noise attenuation. But unless your environment is noisier than about 100dB I would suggest you can adequately protect your hearing without going to the expense and discomfort of wearing a helmet. I have tried a couple of types of helmet, ANR, CEPs, earplugs and a number of different type of headsets in high noise cockpits. The only real benefit a helmet seems to have is in reducing the lower frequencies (less than 125hz or so) which in ear style hearing protection begins to struggle with.
I don't know what sort of flying you are doing that you feel is sufficiently dangerous to wear a helmet. There might be some justification in the argument that wearing a helmet might increase the risk of crashing in the first place (due to distracting levels of discomfort, particularly on warm days). On the other hand if your type of flying has a higher chance of crashing, helmet or no helmet, then a helmet becomes desirable. But it would be worth looking a AAIB reports to determine how often a helmet might have prevented serious injury/death.
Getting helmets and hearing protection right is a real headache!!