The front corner of my car got hit when an idiot merged into me abruptly. At the last second, they decided to make a left hand turn from the center lane of a 3 lane road. I had a lightweight bumper beam. To fix, I replaced the bumper, hood, fender, headlight, and bent the lightweight bumper beam back to shape. If I had the stock heavy bumper beam on there the whole thing would have shifted to the left and caused who knows how much more damage to the frame rather than just giving way by itself.
The car is fine with the lightweight beams, it was designed to be safe with it as that is what it's sold with in Japan. Like other's have said, our larger beams are just in place to pass the US 5 mph test.