Taken from an attorney's site. I realize that this specifically talks about 18 wheelers but it can easily be modified for a truck with an aftermarket steel bumper.
"Does the truck that you drive have a steel bumper? This seemingly innocent stylistic statement can actually be a serious safety threat. A former mechanical engineer in Texas recently determined that steel bumpers on 18-wheeler trucks are significantly more dangerous than the energy absorbing bumpers that are sometimes installed on these vehicles. Steel bumpers can actually make a truck accident much worse than it would have been had the bumper been removed.
This is because the heavy steel bumpers can almost assist an 18-wheeler in doing damage to another vehicle, rather than cushioning the blow. The auto industry has started moving towards the tendency to use plastic bumpers, composite bumpers, or other energy-absorbing bumper materials so that these car accessories don't cause so much damage in the event of a collision. These other materials are able to absorb some of the impact of an accident, rather than magnify it. Many of these other materials will fold under pressure, whereas steel often won't give in an accident."