I think it is a mixed bag: Some supplements are potentially dangerous, but others (like the ones you mention) can be very helpful. The majority, however, probably have little to no effect whatsoever. As a result, I don't think people should mess around with what they eat without it being subjected to rigorous clinical trials first; though there might be a positive net gain, one dose of something bad can kill you.

In any case, though, believing that something is helpful when it has not yet been tested is clearly irrational. (This is more what I concerned about with Best and Kurzweil.) Selling or promoting something that isn't tested is even worse; it borders on fraud and charlatanry.

Edit: No, let me amend that: it is charlatanry.