Riding in groups: Absolutely.
Riding trails: Absolutely.
Riding in congested traffic: Absolutely.
If I'm out by myself on a lonely road: Sometimes.
To and from the post office and around the neighborhood: Rarely.
If I'm going to get hit by a car at high speed on a lonely road, I'm more likely to die of internal injuries than have the helmet make a substantial difference. However, when you're out with your buddies and touch somebody's wheel it's a different story altogether. Broken ribs, wrists, and collarbones are not life threatening injuries but a head injury very well could be. The helmet basically protects me from being a dumbass and forgetting how to stay upright.
The exercise daily article was interesting. I myself have noticed the drivers here give me more clearance when I'm commuting in street clothes with panniers than they do when I'm in lycra. Maybe I need a wig too.