Besides the stake grinding mentioned above and other attacks that only PoS is vulnerable to, another issue is if there are only one or two exchanges controlling volume, and then an exchange gets hacked. If a hacker gets his hands on 10-50% of a PoW currency it sucks, and it's bad, but he can't attack the chain just by virtue of holding coins. This isn't the case with PoS currencies, where a hacker can basically control the blockchain if they get their hands on a large portion of the supply, either through an exchange or some other means. This led to the Vericoin/Mintpal rollback fiasco a while ago, and almost led to NXT rolling back their chain as well in one of the (many) Bter "hacks".
That's true, but if a person had control of that many PoS coins and corrupted the blockchain he would instantly make his own holdings worthless because nobody would want the coin anymore. Once a hack of that size was announced he wouldn't even have to attack the chain, people would be dumping like crazy until it's dead once they realized it was possible. if he were to buy enough coins to do this he would drive the price to the moon with his demand, very expensive for him and then what, turn it all into a shitcoin with forged blocks? Yes he could destroy the coin but he would destroy himself too, only someone very rich and insane could do it and everyone else who sold to him would profit from his loss before it happened. So I don't think that would ever happen.
Meanwhile you can get control of 50% of the BTC hash with 864165 TH. Miners are about $125/TH now, so for $108M you can control a network with a market cap of $10B. That's a 100:1 leverage, I like the odds of that happening a lot better.