Fungibility in my book means no coins are less valuable than each other. For example, a coin shouldn't have it's value obviated because an exchange doesn't like a transaction.
Like @Bryce kinda hinted at, really just the elimination of the concept of dirty/tainted currency.
Try sending your coins to LocalBitcoins or any gambling site. Coinbase will ban your account for TOS violations. They watch your money, after it leaves their exchange.
Of course, they could easily track it through 2nd wallet, but how many degrees of separation then? If you send to a legit biz who then sends to banned biz, are you culpable?
Hadn't heard about this. Investigated & apparently if you send directly from your coinbase acct to one of these groups, you'll get flagged, not if you send to a personal wallet 1st.
Exchanges need to figure out how to manage decentralized excahnges.. currently all good exchanges are centralized.. so they can do whatever. But the tech behind it isn't good enough.