They do offer protection from motorcyclist coming alongside a moving Humvee and tossing a grenade into the vehicle. From reports I've heard this happens quite frequently.
Sometimes the best defensive posture is an aggressive and alert one. If you go out like you're looking for trouble, they'll wait for the next convoy. The guys who need to worry are the ones that look scared (or inattentive) while they're on the road. Roll out every time like you're itching for contact, and all but the most serious nutcases should leave you alone.