To the gold/silver crowd: Pray tell, how could you use gold and silver?
A great question. Many people would simply respond "because it's real money", but that misses the mark by quite a wide margin.
"Because it has been highly valued historically, and many hope this will continue to be true when SHTF" would be closer to reality.
While it is true that gold and silver are "just metals" and have very little intrinsic value, people have come to value precious metals because of their ability to facilitate trade.
To understand why this is so, one would do well to read "What Has Government Done to Our Money" by Murray Rothbard. Within the first few pages, you will gain an understanding not only how gold and silver came to be recognized as money throughout the ages, but why money is necessary for commerce, and why gold and silver do well to fit the bill.
Barter works, but a uniform medium of exchange is required to support effective and efficient exchanges across a society.
While in a Royal SHTF scenario, beans and bullets might be the coin of the realm for a while, when people are ready to start working with money again they will likely feel more comfortable with PMs.
Also, if the SHTF trigger is that the dollar goes into Weimar/Zimbabwe hyperinflation (this possibility looms larger than most would care to believe), those FRNs that folks have in their safes will serve you better as toilet paper, wallpaper, or fireplace fuel than as a medium of exchange...unless you like Monopoly
Should one eschew having FRNs around just in case? Absolutely not. In a more localized situation (flood, tornado, etc.) this could be a true lifesaver, and will likely be gladly accepted, as it has been in most local/regional events.