Can't really see what you're making a case for - it entirely depends on the effect doesn't it?
Sure, if you just want a mysterious flash from "no where", perhaps as you hold two cards over a flame, then don't show the flash paper (but in fact, flash cotton wool serves this purpose way better as it's lighter, burns quick i.e. genuinely in a flash, and is easier to conceal a small amount of between fingers etc).
But effects where you wrap something in tissue and burn it (i.e. Sankey's "In A Flash"), or write someone's name etc, it's perfectly appropriate to let them see the paper. I do a trick with a stick man drawn on paper who eventually finds a card by being burnt up and ending up on the back of card - the flash isn't "the magic", it just brings a sense of excitement and drama to the proceedings.
Flash paper is ace - I use it loads and it always goes down well - and no one has ever, ever said to me or any other performers I know who use it, "wait a second, that wasn't normal tissue paper, was it?"
Love it when Sankey (who is as guilty of flogging gimmicky magic as the next guy) admits jokingly on one of his DVDs, "amateurs spend their money on the latest gimmick. Pros spend their money on cards, flash paper and mouth coils." So true
