Just think of the high speed carnage two semis could do on the Autobahn in Germany or Tokyo highways (trying to think out of the USA box)! He could have been ROTF's Barricade - so much potential and a great intro then slice!
That rule works differently here since semi trucks aren't commercial vehicles. That wasn't really an Audi though. It was a mock up made to look like one. They did the same thing with barricade, who was a mock up Mustang.
And Motormaster would probably suffer the same fate at the hands of Prime's swords. Y'know, after he took Motormaster's face.
You know, I don't mind the movieverse Devastator, but the more I see ROTF, the more I am convinced that combiners work better when they are in configurations like G1 - box style alt modes and bipedal.