Have no auditors who can prove all this virtual Bitcoin trading on exchanger backed by moneyfull private keys.
When people start to windraw Bitcoins from exchangers MTGOX2 will happen.
So better keep your Bitcoins outside echangers for safety.
https://electrum.org/ if you can not use bitcoin core wallet.
It is true that we should not keep our coin in exchanger wallet. Many hackers looking on exchanger and many people think its safe to store it on exchanger wallet. Better move it to our own wallet if we decide to keep it for long term