Well the reasoning I`ve heard is if the SHTF, "junk" silver is a known quantity as they were minted back when the gov still had some measure of integrity and it was/is common knowledge of their actual content, 90%.
Besides, who would go to the trouble of forging dimes, quarters and half dollars, then aging them.
As such they would be a natural for any barter or black-market trade that sprung up.