It all depends on how you trade, if you are a long-term trader then you do not need to leave the currency you own on the exchange, if you are a short-term trader, trade with great frequency, leaving your assets On exchange is required.
Your right, If your a long term trader leaving your coins in exchange is very risky, some trader experience losing their coins even on some popular exchange. Putting your coins in your wallet is more wisely to do,if your done on trading.
There was no suggestion to leave our coin on exchange sites for long term investment. i think the only thing that we can do to leave our coins in exchange sites for daily trading.
That works the traders must send back their money to the wallet after the trade. Some may lose their coins but exchange sites were giving insurance for them all.