Besides all the problems with decentralized money on a centralized exchange, and yes history has shown there is a big risk, also you have to think about a potential evolution in those exchanges. I am having right now 2.5 btc on poloniex for lending purposes. I thought quite some time about the risk of those exchanges, and although its there i think they became much more mature, then they have been just 1 or 2 years ago. Poloniex was even hacked, once, about 10% of their btc was stolen, but poloniex managed to pay everything back. They dont want to be hacked, and they dont want to steal their own coins because its not easy to get away with that...They want to continue operation, why? Because: they have right now 400,000,000 $ volume (24h), which means they make in a single day about a million with order fees. Thats a profitable and scalable business, you dont want to lose. I can afford to lose all my btc without getting into financial problems, so i would advise anybody! Think of it like this: First Phase: nobody cares or sees a problem with cryptos on exchanges. Second Phase: People became aware, and many professionals in the scene are warning about it. But i think the risk is maybe lower then expected.