The idea has a huge amount of merit, not necessarily for Bitcoins.
Suppose I start a company and decide to issue its shares as a block chain. Instead of miners getting 50 shares each, the block chain would be programmed where I just issue all the shares up front to the proper shareholders, etc. The block chain is used as a means to buy and sell the shares on the peer-to-peer stock market (which doesn't exist yet). Miners maybe get transaction fees.
In such a case, proof-of-stake voting might be fantastic.
But I don't see what relevance it would have to Bitcoins as they are now known.
Company shares sounds like a great application, too. The proof-of-stake alternative seems like it could be a good way to prevent unwarranted outside influence from messing with smaller scale public ledgers like they can with proof-of-work.
This unwarranted outside influence also seems to be why alternative Bitcoin implementations don't seem feasible now - you need enough miners to be on board to secure the network against outside attackers, whereas attackers can only come from within using the proof-of-stake alternative. All it needs is a sufficiently diverse userbase.
This is relevant to Bitcoin as an alternative means of forming consensus on the valid transaction history, or the public ledger at least. But all it would necessarily borrow from Bitcoin is the public ledger contained in the block chain, in order to co-opt its userbase.