It's less about that sort of crash and more about small fender benders. You're likely to get much more bumper damage hitting a car at 5 mph in stop and go traffic in a modern car. (The upside being, of course, you're a lot less likely to snap a pedestrian's leg in a similar hit.)

More bumper damage sure, but even at a couple miles per hour, a car where the bumper bolts straight to the frame moving at a couple miles an hour has to dissipate that energy. I'd expect to see some bent metal. My 82 bronco actually has a foot of steel bars that act as sacrificial bumper mounts so the frame doesn't take that impact.