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.