[darawk]

That's a bit of a mistake. It doesn't rely on inefficiency it relies on economic costs. At the moment, hashing is a form of economic sunk cost. If you invest in hashpower, you can't split it without reducing your probability of success. It's an unforgable economic burden. Proof of stake architectures attempt to do the same thing with the currency of the network itself. There are lots of arguments about whether or not this is truly secure - but anyone on here telling you they know it's not is pretty much an idiot. There are tons of proof of stake coins operating successfully right now. There are no known attacks on these coins that allow you to do anything untoward (if there were, they'd be exploited on one of the many existing PoS coins).