I've driven trucks with them, and without. IMO, the jakes should be mandantory on all big trucks. You're SDL has four nice disk brakes on four tires to slow a ~4,000 pound car. A big truck has 10 drum brakes on 18 wheels to slow down 80,000 pounds of truck, trailer, and load. Going down a 7% grade in a truck without jakes is a white knuckle experience while you tiptoe very slowly down the hill, hoping that you've picked the right gear to help hold the truck back, so that your brakes don't end up smoky charcoal at the bottom of the hill. Give me the same truck, down the same hill, at the same speed, in the same gear, with jakes, and I can ease on and off the service brakes only as needed to scrub off a few mph.

As for those signs, I never obeyed those stupid "no jakes" signs. IMO, keeping my truck, myself, and even more importantly, the other drivers that I was sharing the road with from becoming bent up metal at the bottom of the hill, was FAR more important than waking up some whining idiot stupid enough to buy a condo 15 feet away from an interstate right of way. Also, keeping a proper muffler in the exhaust system, instead of running straight pipes does alot to mitigate the jake brake bark.