Wow -- looks like I'm a day late to this party. Helmets do not "improve safety." This seems to be the basis for the guy's argument, and, as most daily-riding cyclists know, all helmets do is protect your head. There are ancillary effects (getting called a dork, having a place to mount your light, etc.), but helmets only do one thing: protect your head. Helmets don't improve safety. These things do:
Being an aware and experienced rider
Riding on a low-risk facility
Riding with aware and experienced facility co-users
Communicating with your facility co-users with your voice, bell or horn
Signaling to traffic and obeying traffic laws while behaving in a predictable fashion (i.e. "like a car").
Wearing a helmet just reduces your chances of severe insult to the organ you keep in your skull -- NOTHING MORE. Further, the connection of "bicyclists who wear helmets are more likely to be hit by a motor vehicle" and "I really doubt if its necessary to wear a helmet while riding my bike to work on the bike trail" when cars are a factor in the former and are not in the latter... okay, soooo not gonna go there.
The study is spurious at best (the results are observed, not causal). YMMV. :)

