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.
In their defense, they offered up repayment for all users affected through their coins. And just the other day announced that they are doing a full buyout of those coins. So they've done a good job at handling the situation -- far better than most.