Allowing Mexican trucks to deliver in the US makes economic sense, the nonsense is that there is no reciprocity, as I understand it.
For example: Houston, Texas is Mexico's #1 port of entry.
Goods shipped by sea to Mexico arrive at the Port of Houston, are off loaded onto US trucks, and transported to a freight forwarding company on the border. At the F.F.Company the cargo is off loaded and reloaded onto Mexican trucks for delivery into Mexico and vice versa. This border freight forwarding is a waste of money. (every freeking truck going either way, stops, is completely off-loaded and re-loaded)
Let the trucks deliver their goods - both ways.
Unsafe Mexican rigs is a rumor and hearsay, a strawman. This is not some drunken wetback in a beatup Chevy, it is a company with a huge investment in equipment, goods, and operators. A loss is a loss and a Mexican company understands the resistance in the US, most strongly from the unions, and what a setback some kind of foul up would be.
As far a the exhaust and the converters go, they were not creating a crisis of air until about five years ago when the omnipotent EPA introduced stricter regulations. Talk to an aftermarket shop about a better/hot chip for your diesel and he will hang his head. He's got a lot of options for pre-catalytic diesels, zip for diesels with cats. This part of the story is another Obama scam.