Example: You come back online after your node had been off.

You receive updates to the blockchain in a PoS world. This is literally no cost to someone trying to commit fraud against you by staking their coins and creating fraudulent blocks.

You receive updates to the blockchain in a PoW world. You start receiving blocks with roughly the same amount of PoW included in previous blocks. You know that an attacker would have had to completely duplicate the entire PoW hashrate to create this clone of the blockchain. Since this would be extraordinarily costly, you can assume that after 6 blocks (or whatever is the expedience to transaction value trade of you're willing to make) it would be economically infeasible to commit.

Creating the implementation of PoS doesn't take an inordinate amount of time. The reason it hasn't been released is because it's a vaporware of a consensus system. The Ethereum foundation won't publicize this because they can continue to convince new ethereum buyers that Ethereum will solve that pesky Byzantine General's problem. https://en.wikipedia.org/wiki/Byzantine_fault_tolerance