Reasons why I have a louvered tailgate: Hooking and unhooking without having to lower the tailgate.
That's it. Simple. I didn't get it because it looked cool, or saved on fuel. Mine is black plastic, got it used on Craig's list for $125. I put it on in the spring when I install the hitch, and take it off in the fall when it's time to remove the hitch. Just part of the experience.
When I would first hookup my truck and 5er, there wasn't much room between the end of the opened tailgate and the 5er. And if I would come in at an angle to hook up, I would be concerned about the corner of the tailgate hitting the 5er. Not anymore.