Oh i agree they will not offer full protection, but i still think they offer some protection. I suppose it all depends on your luck, how you come off after an accident really. Nothing is a 100% after all.
As Adrian says, I think you'll be hard pushed to find anybody here that would say that a helmet would offer no protection in any set of circumstances, but you need to dig a little deeper. It does seem logical to wear a helmet until you do that digging, after all it's a helmet and helmets=safer, don't they? Once you've done the digging and research you can make a decision based less on emotion and anecdote and make your own mind up.
They have started running an advert on my local radio station I can't quite remember it word for word but it's quite short and goes along the lines of,
"If you're a cyclist and you wear a helmet you have a chance, if you don't you have no chance. Please wear a cycle helmet."
In the background is the sound of a hospital heart rate monitor, as I'm sure you can guess when the fella says you have no chance it flat lines.