yes, we've had several threads hereabouts about the propriety of extending the use of fungible for other than commodities (see in particular the Rumsfeld quote).

but here's a perfect usage that I read tonight that made me laugh aloud (from Stephenson's The System of the World:

[Dappa is demonstrating his facility with language(s), although it has been declared that he "doesn't actually understand any of them."]

And owing to this property of minds, it is possible for me to construct in my head an whole universe of ideas, yet each idea will relate to all of the other ideas in precisely the same way that the things represented by those ideas relate to one another; lo, 'tis as if I have created a microcosm 'tween my ears, without understanding a bit of it. And some of the ideas may be records of sensory impressions, for example, yesterday's weather. But others may be abstract concepts out of religion, philosophy, mathematics or what have you--not that I'd know, since to me they are all a meaningless parade of hallucinations. But insofar as they are all ideas, they are all fungible. Whatever their origins may have been, they are now all con-fused into the same currency, and so I may speak of the Pythagorean Theorem or the Treaty of Utrecht as well as I may speak of yesterday's weather. To me they are all just crackers--as are you, my lord Wragby.