Tin Man I don't think it's exactly the same thing for a manufacturer to not "recommend" you use an additive as to say that using an additive is "not recommended". One says that it's unnecessary and the other says it's harmful.
You say there are no double blind studies indicating that additives are beneficial but one could counter that by saying there isn't any evidence that they are harmful either.
One thing that's important to me, it is an incontrovertible fact that diesel fuel standards, specifically in terms of lubricity and cetane are considerably lower in the U.S. than in Europe (the market our cars were designed for).
As for VW and other manufacturers not "recommending" an additive, you have to ask yourself if you think the lawyers and bean counters at VW would make a decision that would lower their liability if the consequence of that decision would be a slight increase in failures, especially if most of those failures would occur after the expiration of the warranty?
If VW recommended an additive, they are essentially saying that they are selling a car in the U.S. that's not suitable for U.S. spec fuel, this opens them up to tremendous amounts of potential liability including super expensive class action lawsuits. Anyone with a fuel system failure could sue VW and use the additive "recommendation" as evidence that VW knew their cars weren't up to snuff.
Also if VW wanted to recommend an additive, they would need to recommend a specific additives if not multiple ones (otherwise people would be putting snake oil in their cars), which would mean they would have to spend money testing and certifying those additives and for what benefit (how does that help VW the company?)
You can be sure that VW will look out for their own interests even if it harms their customers over the long term.
I will agree that we as owners assume some risk when we put an additive in our tanks, but IMO we need to ask ourselves if that risk is enough to offset the reduced risk that we will have a fuel system failure because we use an additive. For me that answer is not even close.