Make a technical or economic argument.
Meanwhile: http://www.truthcoin.info/blog/pow-cheapest/
You're confusing multiple branches competing for being the most-cumulative-difficulty branch, with multiple distinct cryptocurrencies resulting from a hardforking code change.
Penalization only applies to the first case, but the parent was talking about the latter case. E.g. a miner could mine both ETH and ETC once both adopt PoS.
I don't see any problem in hard forks and being able to stake on multiple chains. Let people hard fork as much as they want and decide, later on, which chains hold value.
How? Who decides?
This is really where every PoS algorithm seems to break down. The whole idea of blockchain is to build a decentralized consensus, and PoS just handwaves it away.
Every user of the system.
>How?
Mostly by taking in consideration the differences of the software from both chains.
It is not true that on PoS you can mine multiple forked chains without being penalized. On the chain that "wins" you can be penalized for mining multiple chains which is clearly bad behavior.
The main problems of PoS that Casper, the Ethereum implementation, is trying to solve are "Nothing at Stake" and "Long Range Attacks". Both problems are being tackled right now and close to be solved.