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

Surely, other things being equal (a prereqisite for such a comment being useful) it makes a lot more sense to wear a helmet if you know you are going to a long distance (more chances of having a crash), faster (more chance of hitting your head if you are doing 20 mph not 2 mph) etc etc.

But there's the whole thing about helmets being useless for fast crashes.

if I am planning on riding 10 metres in a park on the grass at 2 mph the possible benefits of wearing a helmet are tiny.

It would hurt less – something most people would agree on about helmets (pain reduction in slow/small crashes).