I read recently that VW's owners' manuals recommend against additives in their gasoline powered cars, but say nothing about additives, positive or negative, for diesels. I'll have to look at my Golf's manual to verify.
And data that supports the need for additives? I was referring to the data in this thread:
http://forums.tdiclub.com/showthread.php?t=268198
Regarding what VW is doing replacing fuel systems at their cost and not recommending additives, I'm betting lawyers, not engineers, are now making those decisions. Replace fuel systems on what, 1 or 2% of the cars out there or risk lawsuits from a much larger group of owners for any number of reasons. Probably a legal and financial decision, not an engineering one.
I've only read of one (PlaneCrazy), perhaps two situations where HPFP failures outside of warranty were not covered by warranty. Are there more? And PlaneCrazy himself acknowledges his HPFP may have been fine. He was chasing a problem that the new pump did not solve. Failure rate in Canada is much lower than the US, anyway. Perhaps it's the fuel?