The naysayers seem only concerned with fatal or life-threatening injuries when non-serious head injuries - which helmets are best designed for - occur possibly hundreds of times every day around the world.
A helmet can not only prevent death (sometimes) but reduces extreme discomfort and pain in the event your head contacts the curb or something hard when you fall.
It's easy for the helmet-haters to downplay the protection when your head is largely intact, and to point excitedly at its apparent uselessness when your head is smashed open.
You might as well say a bulletproof vest is useless because your head and limbs can still get shot. So why wear one at all? Apparently they'd rather be protected 100% or not at all.