The DOT test isn't much in my opinion. I beleive that in layman's terms they put a 10# head form inside the helmet and drop it onto a flat, then round surface at about 17 feet per second. If there is no penetration into the head form, PASS. There's a little more to it than that, but that's the basics.
Say you lay your bike down at 40 mph and you are sliding head first toward a telephone pole... its your full body weight, not just your head, traveling at about 58 feet per second.
Like a construction helmet will protect you if someone drops a nut, but not if someone drops a crane... the helmet (if it has your name on it) will help them figure out who it is. That's all.
In my opinion, all a helmet does is protect your noggin from small incidental flying objects or scrapes from falling down. It actually just increases the wearers sense of safety, therefore increasing their boldness, which relates all too often to riding above their limits. If helmets were against the law, perhaps people would ride safer. Not that that is ever going to happen. I'm just saying - DOT doesn't mean "ride like a maniac, you're covered."