Main point of importance is that you at all times need to hold your coins offline under your own control. An exchange could experience various difficulties where people might not be able to access their funds, the exchange in question can get hacked, the exchange might put a block on your account for whatever reason, etc. If you want to buy a certain coin and wait for it to gain value, then you should send them to a wallet client you have full control over. If the coin you bought gained in value and you want to sell it, then simply deposit these coins and sell them -- there is no point into exposing yourself to unnecessary risks.