[secabeen]

> For the one off scenario where you don't have your helmet on hand, then it's more of an interesting question: "should I chance it and ride the bike without a helmet this one time?"

This is the issue right here. Habits matter, and getting out of a habit can destroy the habit entirely. If the argument is "always wear a helmet" and you cancel rides because of that requirement, I would think you are much more likely to fall out of the habit of riding. If you can continue to ride when you don't have a helmet, you keep that habit going, and next time, you're likely to have the helmet again, and keep accruing the benefits.