Thats an interesting point. I always wondered why there are so many pickups on the streets in Buenos Aires. Whereas on the countryside it may make sense, they are just not made for urban environment (too big, bad for road safety and certainly also for ecological reasons).I don't think the luxury auto tax applies to trucks since they consider them work vehicles.
Just an anecdotal point: When we had the work done in our house, the "master builder" also had a pick up. For him it made sense to transport his stuff with the pick up. He told me though, that it was not possible for him to buy a small transporter (with a bit more capacity) because that would have brought problems with the sindicato de camioneros (they apparently have the monopoly as of a certain size).