I was knocked off my bike by a car about 4 months ago whilst not wearing a helmet, I had forgotten it that day and had just had an earful from my girlfriend about it.
Both me and the car were doing about 20mph, fortunately it wasn't head on and they just swiped across into me. I went flying and landed square on my chin, at that speed there is nothing you can do to "fall in a better way". I ended up in hospital with a suspected broken jaw although it could have been far worse, if I had landed on the top of my head rather than my face I could have ended up with brain damage - or worse...
With the way I fell a helmet would have stopped me from having a huge graze from my chin up to my forehead.
Helmets do work, it is true that they aren't going to save you in a head on collision with a speeding car but they will help prevent sever head injury in the type of accident that happens commuting to work.
Frankly I think its stupid not to wear a helmet, I was stupid, and it should be law in the UK to wear one. Your head is the most important part of your body and you should do what ever it takes to prevent damage to it!