Ok I just spoke to a paramedic with 30 years experience and his opinion is this.
Yes you should wear a helmet if you are riding quickly, quickly being at above 25 miles an hour. Anything mounted to your helmet at this speed is "f'ing stupid" and this includes the plastic peaks. This is because of the speed you hit the ground.
If you are pootling around at 10 miles an hour then you are in no more danger than if you were a pedestrian, and that also applies to the damage you would sustain while being hit by a car. He believes that although there is a increased risk of being hit by a car if you are on the road it is balanced out by the decreased risk of a heart attack so it's up to you if you wear one or not, he couldn't give a monkeys either way.
In conclusion, he very much believes in the freedom of choice and would rather people go cycling with a helmet than not go cycling because they don't want to wear a helmet.
but you are "farking idiot" if you don't wear one in a fast moving pack or in competitions. He would like to note that he would not treat anyone any different no matter what they wear.
He is far far more concerned about chain guards not being a thing any more and would very much like people to make sure their chains/cogs/gears are ok before setting out, especially if you are planning on going fast.