I guess I wasn't very clear, since I was trying to say something similar to what you just said here, thanks for paraphrasing me more clearly. I guess the only part where I have a slightly different view is that crackpot ideas, even those that won't be vindicated by history, can contain some small spark of an idea that may ignite some other idea which will be vindicated. Science fiction is useful for sparking ideas even when it doesn't directly contribute to science. When someone tells me that Basque must be the oldest human language because the last known Neanderthals lived in the Iberian penninsula, I can imagine a nice science fiction story which would make the statement true even if on a different level I know that the statement is a series of weak or impossible links in a chain of improbable assumptions on a subject that is a magnet for quacks.