When you are trying to use a blockchain as a store of value or currency, as Bitcoin does, then the cost of Proof of Work mining may be acceptable for the reasons you mentioned.
However, being used as a currency is not the only thing that a blockchain can do, and many newer crypto projects are attempting to do other things. For example, you can use a blockchain to act as a company (a 'Distributed Autonomous Company', or DAC). The DAC can provide services to users, in exchange for fees, and attempt to make a profit. For example, the Bitshares project functions as a company that provides an exchange on the blockchain, in which you can trade assets such as gold, dollars, bitcoin, etc.
When you are attempting to run a company on a blockchain, profits matter, and that means that you need your fees generated from customers to exceed the cost to secure the network, and that means that you need to use a more efficient security algorithm than proof of work. Various Proof of Stake algorithms cost much less to maintain. If the goal of your blockchain is not to represent all of the money in the world, but rather to be an autonomous company performing a certain service to users, then maybe you do not need to go 100% all in on security, perhaps it makes more sense to try and make a profit using a cheaper consensus algorithm.
Finally, there are now a variety of different Proof of Stake algorithms. Not all of them suffer from the vulnerabilities of the original proof of stake, in which you can use old keys which used to control many coins in order to attack the network. For example, Ripple (Ripple Consensus algorithm), Bitshares (Delegated Proof of Stake), and NXT (Transparent Forging) algorithms do not suffer from this weakness, as far as I know.
Full disclaimer: I work for Vitalik and the Ethereum Research Team.
Here are a couple of reasons for why we are ultimately eschewing PoW:
(1) We are pushing to get blockchain updating down to 3 seconds. You really can't do this with PoW, since 3 seconds is our budget for our network overhead.
(2) A single block chain doesn't scale well - we're pushing for protocols that involve multiple blockchains processing transactions independently. The current proposals all rely essentially on PoS based on the central Ethereum Blockchain. Proof of Work isn't generally...
Vitalik says cryptocurrency developers should "reach out to LessWrong more", in order to not be ignorant of advanced computer science and game theory topics.
https://blog.ethereum.org/2014/12/31/silos/
I think Vitalik raises several major important points, namely:
* Bitcoin's $600 million wasted on electricity yearly in order to secure the network is a huge problem. Various Proof of Stake algorithms are attempting to improve upon this. The ideal solution would be one which combines the low cost of the Proof of Stake algorithm, with the high security or Proof of Work. I not enough of an expert to be able to say which solution is best, but both the Ripple consensus algorithm and the Bitshares Delegated Proof of State algorithm look promising. I am interested to see what algorithm Vitalik will choose for Ethereum.
* ASIC miners are a problem due to the resulting centralization. They have also lead to increased centralization in Bitcoin, as the network is now increasingly controlled by a few large mining pools.
* Blockchain technologies have a great potential to change the world, and solve governmental and organizational problems that society is facing. Beyond simply revolutionizing money, blockchain technologies could also be used in the future to support prediction markets, voting/consensus building, exchanges, etc.