Making comparisons to other forms of head injury (like from falls) is a terrible argument. You need to look at per-capita incidence of head injury. Not everyone rides bikes, but everyone walks on stairs. You need to look at the numbers within a demographic. If we looked at the incidence of bicycle-related head injury per 1000 bicyclists and compared that to the stair-related head injury rate of the same 1000 bicyclists, I guarantee more people from that group would have gotten head injuries from their bikes, rather than stairs. However, if we took a group of 1000 85 year old people and got the same numbers, we'd find that overwhelmingly they get more injuries from stairs than bikes. But that is because they are old, losing their balance, and don't ride bikes. Hence, their numbers are meaningless for a discussion on bike helmets.
If you are going to discuss helmets for bicyclists, stop pulling out numbers for people who aren't bicyclists, since they aren't relevant. I think that the statement, "Bicycle related head injuries are a common form of head injury among regular cyclists" is a true statement. Hence, I think helmets are useful.
I'm a libertarian on most issues, and so I don't support helmet laws, smoking in bars laws, etc. However, that doesn't stop me from thinking that not wearing a helmet is a foolish idea based more on macho-ness or vanity than actual statistics or logic. Now, its a 'free country' so you can be macho and vain, but I hope you choose not to be.
peace,
sam