I've heard people claim its because backing money with gold (or even making it OUT of fucking gold, hard to tell, at times) would somehow magically make our money non-fiat money. Of course, this is bullshit, because all money, no matter what its made out of or what backs it, is fiat money: its value is only what a person believes its worth. For example, I likely view a single dollar as far more important than Bill Gates does, ergo, it has more intrinsic worth to me than it does him.
If these retards would stop and think for a moment, they'd realize that the only "libertarian" way of trade would be a return to the barter system, where people trade only for what they need and/or want, anything could effectively be currency, and establishing standards would be a nightmare for anything beyond a local scope.
Basically, they don't really understand why humans invented currency in the first place.