What can be genuine benefits of double spending the same coin other then being misused like the way its been here?
Double spends aren't having any benefits. Double-spending is the result of successfully spending some money more than once. Bitcoin protects against double spending by verifying each transaction added to the block chain to ensure that the inputs for the transaction had not previously been spent.
For one, its only called a double spend, the value has not actually been spend twice. The first transaction became invalid and thus the same coins have only be confirmed once.
Other electronic systems prevent double-spending by having a master authoritative source that follows business rules for authorizing each transaction. Bitcoin uses a decentralized system, where a consensus among nodes following the same protocol is substituted for a central authority.
Bitcoin has some exposure to fraudulent double-spending when a transaction is first made, with less and less risk as a transaction gains confirmations.
After one confirmation you would need to find a competing block for the one that has confirmed the transaction. This is only possible with certainty if you hold over 50% of the networks hashpower. There is no "less and less risk" its a significantly drop from unconfirmed (can be double spend with a few simple steps as long as the conditions are right) to 1 confirmation (you need more hashpower than the rest of the world).
Traders and merchants who accept a payment immediately on seeing "0/unconfirmed" are exposed to a double-spend occurring, if there was a fraudulent attempt that successfully communicated one transaction to the merchant yet communicated a different transaction that spends the same coin that was first to eventually make it into the block chain.
Merchants can take precautions (e.g., disable incoming connections, only connect to well connected nodes)
That does not help, what you have in mind is a sybil attack. A sybil attack can be used to make a double spend simple, but its - again - very resource heavy (you need enough full nodes to fill all connections of the merchant) and would be hardly worth the effort for transactions commonly accepted without confirmation.
The best way to mitigate a double spend for a merchant is to constantly rebroadcast the TX or as e.g. bitpay does have direct deals with big mining pools.
to lessen the risk of a race attack but the risk cannot be eliminated. Therefore, the cost/benefit of the risk needs to be considered when accepting payment on 0/unconfirmed when there is no recourse against the attacker.
Here's
an example of how macbook-air (owner of F2pool) had successfully double spent $10000 against OKPAY!
They used a very specific network state, the 0.8 fork, this is not something you can repeat daily. The window was small and known in advance.
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Was only an issue while there were two chains; ie, for a couple hours last night. The possibility was well-known, which is why it was recommended that merchants, exchanges, etc, stop taking payments/deposits until the issue was resolved, which it now is.
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What can be genuine benefits of double spending the same coin other then being misused like the way its been here?
Increase the fees on an otherwise stuck transaction. Usually you want double spends to be hard though, because if its too easy you cant accept unconfirmed transactions and bitcoin would slow down even further and be useless for small transfers to e.g. buy a coffee. No one is willing to wait 10 minutes for a coffee.
