If bicycle helmets were essential for causal-speed, safe path, everyday bicycling, the Dutch would use them.
Interesting fact: in the US, riders without helmets receive more average buffer space from road vehicles than those with.
If you're downhill mountain biking, wear a proper helmet. If you're racing in Manhattan, wear an equally suitable helmet. If you're leisurely riding on a wide, empty side road in a residential area and cycle often, it may lend more placebo overconfidence to the rider than it confers necessary protection. It seems like gearing-up in knee and elbow pads to go for a simple walk.