Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

^ A lot of people seem to just wear them on long or fast rides -- isn't this just admitting that they're effective? So why not wear them all the time?

It's likely that they don't vary their perception of the benefit, it's that the costs (inconvenience, sweat, helmet hair) are discounted more on a long training ride rather than a 5 minute trip to Sainsbury's. Since the costs are largely fixed, the cost:benefit gets much better as the ride duration increases, even without the further discounting which applies to the different type of ride.