It all comes down to quality of the road, line of sight, and entrances. There are a lot of places where it makes sense, with enough clear space. But then again, a speed limit should be the speed at which 85% of drivers are comfortable driving. So if you have a 55 MPH road with lots of people doing 70, maybe make it 65, adjust some of the clearances and run-off, and be better off for everyone. Especially, because then people expecting one sort of speed, aren't blindsided by an entirely different speed.