Dave has over 60,000 miles and more experience with horses than most of us could ever hope to achieve in a lifetime. However, accidents happen - and Dave is only one of many experienced endurance riders that I know that have received major injuries that could have been prevented had they been wearing a helmet. You never know what could happen at any time, no matter what you're doing or how many times you've done it before. Plus, particularly concerning endurance, there is a lot of chaos and unfamiliar circumstances for both riders and horses, and you are usually a long way from the help you'd need if anything did happen. Please, if you think you don't want to wear a helmet, think twice about that choice.
On a side note, many riders I know are member of the Life Flight network, in case they do ever need urgent care. Something else to consider as an endurance rider.