Quite a while back in a thread about front air dams playing a role in fuel mileage I was basically told by some that I was full of it. All y'all can kiss my grits, see video below. So there!
You will also see light weight belly pans on many vehicles that do not function as skid plates. Their primary purpose is to increase aerodynamic efficiency of the under carriage. A secondary benefit is less road grime and debris accessing the engine.
Air dams have their place, but I do feel they impede true 4wd use. If you look at most Toyota pickups used off-road, the air dam is the first thing that is removed by the owner or terrain.