is fungibility of ALL bitcoins really important though?
Consider that you own 50,000 bitcoins. If 1,000,000 bitcoins were found to be in the hands of ISIS, and blacklisted, what would that do to your bitcoins? Well, how would that be different, from 1,000,000 bitcoins that are simply lost forever? That some bitcoins are no longer fungible/available doesn't harm your coins in any way. In fact, it would benefit you, since the less coins in existence, the more valuable your coins are.
The only thing would be if blacklisting of coins is backtracked (dunno the term so I invented one). What I mean is, say you did business with a company, and your coins are connected to their addresses. That company then proceeds to scam people and engage in terrorist activity say a year later.
In such cases, I think that there would be laws that would prevent your coins from being blacklisted. It shouldn't be difficult to see that your coins were obtained before the time they engaged in illegal activities, or at least before the public was aware that they were engaged in illegal activities. In such cases, I would think that there would be laws preventing those coins from being blacklisted.
So unless you are planning to do something illegal yourself, other people's coins being not fungible doesn't harm your coins. It makes yours more valuable.
Because it will be YOUR bitcoins one day that will be blacklisted. If you don't protect the fungibility of all coins - YOURS will eventually come under scrutiny.
Why? Because of "association", because who ever paid you had 'bad' coins, because you donated to some political cause, or because police required them to be under civil forfeiture, because someone challenged you (unrightfully) in court, because you are *insert whichever race randomly unpopular at the time*.
Political persecution IS A REAL thing, even (and very much so) in the Land of the Free. (here's a contemporary example:
http://en.wikipedia.org/wiki/IRS_targeting_controversy)
Laws and societies change. Sometimes for worse, and it seems we are head-first into one of those worse periods these days and years.