None of the posts in this thread so far have really explained the problem.
Bitcoin's 0-confirmation transactions have three major flaws:
1. They are too slow for certain types of microtransactions such as clicking the mouse to access some Internet resource. No one wants to wait 5+ seconds every time they click the mouse. The reliability against a double-spend increases as propagation of the transactions across the full node network completes. This depends on the weighted average of hashrate and the slowest latency on the network. Thus the way to improve this speed is to centralize mining, which is probably why ButtCON has been getting faster as
mining has essentially become entirely centralized and only an illusion of decentralization remains.
One might argue that no one will bother to double-spend such small valued instant microtransactions, but in fact these are the most vulnerable to double-spending because the the payee can't delay the access for many seconds, the payer can't get caught in the virtual act unlike retail and because there might be an incentive to sabotage an Internet merchant or service (e.g. competing social networks attacking each other covertly, politically/religiously motivated attack, etc).
Some references:
https://www.reddit.com/r/btc/comments/3ze0sz/why_bitcoin_0_confirmation_transactions_are_safe/https://chrispacia.wordpress.com/2015/11/29/on-zero-confirmation-transactions/https://blog.blockcypher.com/from-zero-to-hero-bitcoin-transactions-in-8-seconds-7c9edcb3b7342. The Bitcoin block chain can't scale to handle the level of transaction rate that microtransactions would require without being centralized.
3. It seems the core developers may be intent on implementing Replace-by-Fee and destroying 0-confirmations, although RBF will apparently be opt-in so it wouldn't destroy all 0-confirmations. Although RBF seems to have a legitimate use-case for being able to prevent transactions from getting stuck if the fee ends up being too low in a free market competition for setting transaction fees:
https://bitcoinmagazine.com/articles/bitcoin-core-developer-jonas-schnelli-explains-controversial-transaction-replace-by-fee-feature-1454343556Also
the collusion between Blockstream and the Chinese mining cartel potentially breaks 0-confirmations by not increasing the block size sufficiently to handle all transactions that have a sufficient fee, which by definition is not spam. Which btw may incentivize payers to use RBF. Perhaps Blockstream may be trying to push towards
Lightning Networks, which is also flawed and can't scale decentralized either.
Note many people may think that instant microtransactions are irrelevant, but when that ends up being the future economy of the Knowledge Age and displaces the retail and tangible economy by several orders-of-magnitude, then the dinosaur (Model T) will be Bitcoin.