The fact that you trust your exchange is great! Clearly without the trust of their customers, they could not operate, but Mt Gox seemed secure to most people as well. Storing your coins in the exchange instead of a cold wallet will always represent a higher risk. As long as people are realistically aware of the risk and are willing to accept it, then I think you're fine. 2fa is your best friend, but it sounds like you know that. If you are handling an amount of money that you would be a bit more than upset to see disappear tomorrow, it is probably best to store it offline.