Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

He said himself he is shit at riding off road. To me riding off road is not riskier no, not for fatality/near fatalities. If a car travelling at 40 mph hits me then I think that is going to be a much higher consequence than me sliding off the trail at 10mph onto some mud/forest floor.

This is true but the forces produced in a 10 mph fall off on the mud are the type of force cycle helmets are designed to protect you from. This is the area where cycle helmets work well. The forced generated by a car hitting you at 40 mph are well beyond what a cycle helmet is capable of negating to such an extent that the chances of it affecting the outcome of a crash are very small. An oven glove is very good at protecting your hand when bringing a cake out of the oven but would not change the outcome of trying to pick up a piece of near molten rock recently jettisoned from a volcano.