There have been studies that suggest most preventable deaths in aviation are the result of head trauma. That certainly makes sense given your head is probably the most critical and most vulnerable part of your body in an impact. While I'd always suggest starting out by investing in good shoulder harnesses, they don't guarantee your head will stay away from the panel in a crash, so a helmet is potentially a layer of safety in any aircraft.
You have to decide what the cost-benefit is for you, but operations such as bush flying could put you at elevated risk and might make a helmet worth it. I've even mulled getting one for myself, and I'm usually not doing anything more than run of the mill GA flying. I always wear a helmet while riding a bicycle and almost always wear at least an inflatable lifejacket while boating. I just feel like it's stupid to get into a survivable incident and then be seriously hurt or killed because I wasn't wearing the right equipment. Is a helmet for recreational flying overkill, or is our flying culture just more tolerant of that kind of risk? I'm not sure.
There are tradeoffs with every layer of safety. We might prevent a few fatalities a year if we all wore nomex flight suits and gloves, military-style boots, and helmets. While some pilots do choose to wear that, many of us are content to simply wear non-synthetics for fire resistance and leave it there. It's a decision we all have to make but we ought to make sure we don't let anyone make that choice for us.