I think the most convincing example of fungability necessity for me was when some bitcoin exchange did not accept bitcoins from someone, because allegedly those bitcoins was connected to some dark market activity.
It's not like money should be fungible for criminal operations, but its a good example when some bitcoins != usual bitcoins. This is impossible with monero.
But those coins can go to mixer and come out a different ones.
Kind of off topic but this thread has been slow so I'll jump in as well.
When the government decides to make it mandatory for business to not accept tainted coins, which it will inevitably will, those that have gotten them even by legal means will lose them. This is the most important aspect of fungability. Using a mixer with good coins right now is as retarded as it gets. If the canary in the coal mine (exchange not accepting tainted coins) was not enough to convince then those that lose deserve to.
The only reason this has not made a bigger splash is because those invested in BTC refuse to propagate the fact their coin is compromised. The writing is on the wall.
You bring up some very important points to consider.
There is a very good possibility that exchanges and merchants that are regulated that accept bitcoin will seize your bitcoin if it is tainted from a previous "hack/scam/theft/etc".
If that were to happen, which I think it will, there would be a scramble to get "clean" coins. Lots of users would be turned off as they would probably be buying coins that are tainted and when they go to cash them out they may have some issues.
But in terms of in person transacting I can't see how this would be an issue at least at first. There could come other scenarios that make in person transactions stray away from accepting tainted coins for transacting.