Accidents may be less probable, but if you happen to be one of the unlucky ones who does have one you could still die or suffer severe brain damage, so why not just wear the helmet? Sure it makes you look a bit goofier, but that's a small price to pay to protect your melon.
I do a lot of downhill mountain biking, snowboarding, etc, so I'm not one of these people that tends to be overly worried about safety, but whenever I get on wheels or something that can easily slide out of control I wear a helmet, period. I'm (pun intended) painfully aware of the amount of impact force a rotational component can add to an otherwise relatively low-speed, low-altitude fall.
Maybe the drivers are more cognizant of bikers in the Netherlands vs the US, but I still wouldn't trust them, they are not infallible and it only takes one of them one fuckup to mess me up. A helmet is a very effective (though non-perfect) fail-safe for this that costs virtually nothing (amortized over time of use before it is needed, hopefully never) and has no real downside other than pure cosmetics.