There's no legal case against anyone at any time, in any situation or case, where a rider wasn't wearing a helmet - in areas where the wearing of one is not mandatory by law. Here, it couldn't be used against me.

Should you wear a helmet? Or replace a damaged one? Yeah, if you're likely to hit your head on something because you don't look where you're going. In fact the really clumsy should just wear one 24/7 regardless of what they're doing.