Let's see:
#1. Helmets do have prevented head injuries in some actual accidents.
#2. Helmets make many or most of the wearers feel safer.
I think Effect #2 greatly outweighs Effect #1.
The main role of helmets seems to be making the wearer feel safer (or keeping their parent/spouses from worrying for them), without actually making them safe in many situations.
I think that this argument is too easy. It could be a good argument against helmet laws, for example, but for an individual, it's just not necessarily true.
Helmet effectiveness is pretty static. It could change slightly with different designs, but with statistics etc...we could determine how effective a helmet actually is.
Risk compensation is not in the same category. Yes, the problem of risk compensation is real. People feel too safe because they wear a helmet, They cycle dangerously, and get their arm broken, or worse. I agree with many others that they take up WAY too much space on the cycling safety radar and people put too much stock in them.
If people were better informed, about helmets, about their own psychology, about cycling safety, it's possible that they could not have a false sense of security based on wearing a helmet.
It might be a good argument against mandatory helmet laws, but not weather people should wear helmets at all. I have no control over the effectiveness of my helmet (except choosing between different designs) but I do control my behaviour, and can even compensate for sub-conscious behaviour.