Risk of death is a only one metric for evaluating helmet use, and it's arguably a poor one since it is relatively uncommon (partly thanks to medical intervention). In addition to death, bicycle accidents can also cause varying degrees of physical and mental debilitation. One of the most important things to examine in the discussion of helmet use is the risk of traumatic brain injury (TBI). Helmets do help there, significantly [1].

From the cited article:

"[...] One third of the cyclists who are admitted to hospital with serious injury after a traffic crash are diagnosed with head/brain injury. Approximately three-quarters of these cyclists sustain this head/brain injury in crashes not involving a motor vehicle. As many as nine out of ten young children who sustain head/brain injury, do so in crashes not involving a motor vehicle. In the majority of cases these are bicycle-only crashes.

Research has shown that a bicycle helmet provides protection against serious head and brain injury. The most recent estimate (Elvik, 2011) indicates that the risk of head injuries is 1.72 times higher for cyclists not wearing a helmet than it is for cyclists who do wear a helmet, with a 95% confidence interval of 1.33-2.22. For brain injury the risk seems to be 2.13 times higher, with a 95% confidence interval of 1.33-3.45. [...]"

There are many other studies, with the overwhelming majority pointing to helmets being significantly protective against TBIs:

http://www.ncbi.nlm.nih.gov/pubmed/?term=bicycle+helmet+trau...

If you're going to be hit by a truck, there's only so much you can do. In the more likely scenario where you're falling to the ground and will hit your head, a helmet will most likely help your outcome. I for one don't want to live with a debilitating and irreversible brain injury, so I always cycle with a helmet.

1. https://www.swov.nl/rapport/Factsheets/UK/FS_Bicycle_helmets...