No safety equipment, no matter how well it meets anyone's standards, can protect you completely from all possible hazards. Helmets do protect you to some extent, even minimalist ones. Likewise for gloves. Any helmet reduces surface abrasion, and reduces impact acceleration, and therefore can be good for some people and purposes. I.E., they may not prevent the bouncing brains, but they do reduce that - except in sports where the wearing of helmets has caused people to be more careless of their heads. (Which is why introducing helmets to a sport, as I mentioned before, invariably increases injuries - a statement you will find in any athletic training manual. As an example, more injuries / hour of play occur in American football than in rugby.)
I once took a bike ride, wearing a very lightweight minimalist (foam only, Giro brand) bike helmet. I awoke in a hospital room, post-surgery. (No memory of what had happened - hospitals deliberately give you drugs during anesthesia designed to destroy your short term memory, so you won't remember the pain.) The helmet was in pieces on the bedside table. The helmet hadn't protected me completely - but I believe it probably saved my life. So if someone tells me that helmets that don't pass ANSI construction worker protection standards are worthless, I won't believe them.
I personally happen to believe that fall and collision training are more important to preventing injury than safety equipment. But protective gear, whether or not it meets anyone's formal standards, can still be very useful, especially to those without such training. I personally don't wear a helmet (for figure skating). But I almost always wear gloves, long sleeve shirt or jacket, and long pants, which provide a lot of abrasion resistance. Abrasions are a lot more common than concussions, and I know from experience that they can lead to severe infections. And I have used helmets while bicycling, whitewater kayaking, rock climbing, and caving.
BTW, War and other contact "sports" aren't just a recent trend.