Enough cannot be said on this issue really and for sure people will still leave their coins on exchanges for reasons ranging from convenience to cost of transaction fees, moving the coins in and out.
Really there are trusted exchanges, and one can afford to trust to a certain extent, but even though,leaving coins in there could be risky, but yet understandable
When it becomes foolish is when you leave your coins in an exchange that's yet to gain any sort of ground in the cryptocurrency world, then the risk is more and it's best you move them either to your wallet or exchanges with the best form of security