To the people worried about fungibility, they need to remember that this blacklist via tagging is not centrally enforceable. It is an opt-in tool that exchanges can use to avoid stolen XEM at THEIR discretion.
If coins may be blacklisted, they are not fungible. It has nothing to do with centralization, opt-in, or discretion.
There mere fact it is possible for anyone to blacklist coins proves the coins are not fungible.
Truly fungible coins cannot be blacklisted. Look into the Monero literature for more details.
Fungibility is not binary where a currency is either 100% fungible or 0%. Yes Monero is much more fungible (not 100%) and I would even say it's the most fungible of all blockchains. It wins in the privacy category but that is not NEM's market. NEM is a smart asset platform that is willing to trade fungibility (some but not all) in exchange for transparency.
The point is that this tagging system is NOT a blacklist but a tool that can be used to track stolen assets. Yes this does in fact reduce fungibility but not as much as a true blacklist such as the case with Ripple or the ETH hardfork. I personally think this approach provides a good balance and is aligned perfectly with NEM's value proposition.