I don't agree with that. Just because the factory didn't see a need too, or wasn't legally obligated too, install Cats, or any other emission devices, doesn't mean you should just say, "F it". Whether, or not, emission tests are required in any given area, I still believe we all have an obligation to make sure we don't, knowingly, pollute the environment. Basically, just keep up on your tune-ups and regular maintenence, and take care of any obvious problems. It can get embarassing for yelling at someone for accidentally spilling oil on the trail, and not cleaning it up, only to have them call you on your grossly polluting tow rig. Also, just because a tow rig might be a diesel, doesn't mean it's ok to just allow it to billow out the black smoke, and soot, everytime you get on the pedal... adjust the injector pump, check/replace the injectors, adjust the air/fuel mixture, or whatever it takes, to lessen the amount of soot