I think it's worth wearing a helmet, if you fall off at speed, your side, legs arms etc, will most likely take most of the impact but the helmet may save you from a smaller knock you get as well.
Saying that I rode to school without one, and used to ride a few miles to work without one when I lived in the uk. For longer rides where I was only riding the bike I would wear one. Just get used to it, feels strange without one now.
I don't wear a helmet and fell of twice so far (both times last year not long after buying the bike)
once at 16mph ish trying to turn a corner at to shallow an angle and the back wheel skidded along a raised curb whilst the front end was going a different way.
Didn't come close to hitting my head but scraped my palm pretty bad.
second time at the park when it was raining going about 12mph breaking for a bend and the back wheel must have slipped on wet moss or leaves as the bike ended up underneath me and pretty much the same damage done.
I can see how a helmet can help you if it's your own fault like the above but if it's an accident with a motor vehicle then I don't see it making much difference.
If I went speeding down descents at 30+ mph then I would likely wear one or if I was an actual commuter
Proper kevlar motorcycle armour might make a huge different but I don't see the helmet brigade wearing them instead they neglect the other 90% of their body that will likely need skin grafts in any accident where a helmet saved their life.
Usually the biggest voice about helmet safety isn't actual cyclists anyway it's other people who perceive a danger that is wildly exaggerated by the media and government.
with loads of other cyclists because it is the easiest way to get to london bridge.