Just in case any of you were unaware, last mont Dave Rabe, a legend in the endurance world, one of the top lifetime mileage earners, and a personal inspiration of mine, had a major accident at an endurance ride. You can't miss Dave - he's always wearing his cuttoff shorts, tank top (if he's not shirtless!), and ball cap - never a helmet. As he was getting on his horse, it started bucking and Dave ended on the business end a traumatic brain injury. It was a very scary few days in the endurance world as we awaited news of his condition. After hospitalization and many tests, he is now going through rehab and recovering. You can read more about Dave and get updates on his condition here: Dave Rabe « XP Rides
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.