IIHS has released a report on the poor state of under-ride protection on American large truck trailers. If you watch
this video, I promise you you will feel queasy. In it, a 2010 Malibu is crashed into the rear of a U.S. spec trailer at 35mph. All of the Malibu's excellent front crash structures slide under the rear of the trailer, as the guard buckles and folds under, and the rear of the trailer hits the dummies in the head.
Adding beefier rear guards to trailers will cost a little cost, and a little weight, to each trailer. But seeing as there are hundreds of car-trailer accidents each year, I think it is probably worth it.
Another issue, which IIHS doesn't discuss, is side protection for trailers. European trailers have side under-ride guards as well as rear.