Bitcoin is perfectly fungible. Anyone who claims otherwise is confusing economic concepts.
we already had this discussion.
maybe bitcoin is technically fungible (because any coin can always be send an mixed) but practically you have problems when using coins which where stolen (eg bitstamp hack) or why do you think new minted coins are sold with a premium?
Here's an economic concept: the market places a 10% premium on freshly minted zero-taint BTC over 'gently used' coins. Good luck with your stolen Evolution coins!
Bitcoin doesn't meet the highest 'can't be evil' / 'can't be non-fungible' technical standard because the code may be modified with black/white/grey/red lists of forbidden/allowed coins/taints.
So no need to even get into the more significant (but more general issue) of socioeconomic interchangeability.