It's widely known, and manufacturers will tell you, that a bull bar will result in significant fuel economy impact, anywhere from 6-10% depending on where and how fast you drive. The physics of wind resistance combined with the added weight causes a dramatic increase in rolling resistance of the vehicle. If you get 20 MPG, that means at best you'll get 18.8 MPG with a bull bar. If you drive 80,000 miles a year, at $3.50 a gallon, that's $900 a year, plus the cost of the bull bar, and the increased pollution, all for the privilege of looking cool.
The biggest problem with most bull bar designs is that they bridge the "crumple zone" with a rigid structure, so the vehicle cannot crumple in the manner in which it was designed. The "crumple zone" is all but eliminated. This, of course, transfers the crash forces into the cabin of the vehicle and he driver, rather than the chassis. Even low speed collisions usually transfer the crash forces deeper into the vehicle structure, actually causing more damage than would occur without a bull bar. A bull bar can cost you $900 a year to run, and it'll usually double or triple repair costs if you hit a large animal.
Also, because a bull bar changes the crash characteristics, air bags are often rendered useless because they tend to fire too early or too late.