Poloniex is one of the greatest prizes in hacking at present. Lord knows how much money is stored on there. I'm sure they're extremely careful but another hack is only a matter of time.
It's already happened and somehow it'll happen again. Considering the returns on offer there's no way I'd bother myself.
Or better. They could do the big Verning and run away with all the BTC and other coins. Its so easy to get away with it since altcoin exchanges arent regulated.