As known, 51% attack is when a miner or mining pool controls 51% of the computational power of the network and creates fraudulent blocks of transactions for himself while invalidating the transactions of others in the network. With a PoS, the attacker would need to obtain 51% of the cryptocurrency to carry out a 51% attack, which becomes very costly with the growth of the network, let alone it will require quite some coding both in daemon and in the wallet.
Let's assume you need 40 confirmation to deposit to exchange. Even if you have a group of 75% of the stake that signs "official" and "alternative" chain, chances that 75% can grow a heavier chain than 100% are quite low. The 75% are much better doing classical attack — leave the other 25% alone to build a weaker chain. Eventually it boils down to convincing more than half of the anonymous stake to perform an attack that will in fact discredit the chain and the coin. It's neither feasible nor reasonable.