If I still rode offroad, I would use a lid for that every time. Conditions in the dirt are right for a helmet to have a solid positive effect. You are usually not going so fast as to exceed the strength of a lid. The stuff you are most likely to run into, tree branches, dirt piles and brush usually have enough give that a lid will serve its purpose very well.
Out on the streets and highways, the environment is different. You are most often traveling at speeds where a helmet isn't going to make a whole lot of difference in a crash. Because of the reduced peripheral vision you get with many helmets, the fatigue from the weight, heat and airodynamics of a lid, there is an increased chance of not seeing a hazard until it is too late to avoid it. Because sound travels at a different speed through the material lids are made of, it is usually difficult to tell exactly where a sound is coming from, so you have to look around to see just where a hazard might be. For an example, a few years ago I heard a truck tire blow out, but couldn't tell where untill the one in front and one lane to the right started shedding road snakes.
What it all boils down to, on pavement, helmet or no makes much more difference to the news people than to the rider.
As for not getting in a accident if you stay home, even if you spend your life hiding inder the bed, there are health hazards lurking in the dust bunnies and what if the bed slats slip or break?
