Non-denominated “forever” stamps have an inherent value, because you can be assured that whatever future rate hikes occur, they will always have the value of a letter being delivered. Stamps with a dollar/cent denomination on them, on the other hand, don’t have an inherent value because you can’t be assured that their face value will buy a delivery in the future. They have no more inherent value than the dollar/cent fiat currency in which they’re denominated.
I don’t know about domestic Confederate counterfeiters, but heavy northern counterfeiting of Confederate money badly damaged the Southern economy. Note in the story, the counterfeits were often better made than the real thing.
They can still be used. You just might have to add a few cents of additional stamps. So yes, they have a value.
I like that it lists “Broken Banks and Fraudulent Institutions.” Two ways for a bank note to be worthless without being counterfeit.
I’m not saying they don’t have any value at all; I’m just saying the value is no more intrinsic than that of the currency in which they’re denominated. Take the rate increase from 46 to 49 cents. Sure, you could still use an old 46 cents stamp, throw in another three cents, and you’d get delivery; but the same was true if you had 46 cents in cash - you could throw in another three cents and buy the delivery of a letter. If U.S. currency is fiat money, then so is a stamp denominated in U.S. currency.