It offends some people when others even wear a helmet. Seriously, I have had a grocery store cashier launch into me about my helmet, when I was otherwise dressed in completely non-bike clothes (and if she saw my bike at all, which is doubtful, it would've been my inexpensive, upright-seated commuter).

Just the other day DH and I (again dressed entirely in street clothes, but wearing helmets) were trundling down the MUP on an errand. We rarely exceed 10 mph on that path, and we normally slow down to 5-6 to go around pedestrians. A couple in their late 60s or early 70s were coming out of an intersecting unpaved path, without looking either way on the MUP. We dinged our bells multiple times, honked our horn, slowed down below walking pace, maybe 3 mph, totally with no acknowledgment from the couple, who continued to move slowly into the center of the lane. We finally rode into the dirt in the far side of the MUP to pass them. "Nice bells," said the woman as we rode past. "I just didn't want to run you over!" I hollered over my shoulder. "Oh yeah, you're SERIOUS cyclists," sneered the woman.

It doesn't matter how short the ride, I don't go out without my helmet. I don't really understand why it threatens people so much, but I'm not about to stop wearing it.