Just wanted to throw in the comment that because I ride alone, in Nat'l 
forest trails, usually far off the beaten track, I ride with a helmet, 
carrying a small survival kit, etc.  I run into the forest rangers on a 
fairly regular basis and have heard comments on at least three occasions 
that they wish more "pleasure" riders would wear helmets, that they've 
had to rescue over the years many, many riders that were injured more 
seriously than necessary because of no helmet, including a number of 
fatalities (we have alot of high cliffs and deep canyons around here).
Not passing on any judgments on anyone's decisions, just throwing in 
what I heard from the rangers.