It's nothing wrong with trading, and keeping a small tradeable % in an exchange.
But when I see people using exchanges as banks, some guy wired 1 million $ to an exchange, and left it there after seeing how much "safety" the exchange guaranteed, I mean: 2FA auth, HTTPS, e-mail confirm on withdrawal... Wow that must be ultra safe, what could go wrong?
Just ask all the exchanges that used the same security protocol and still got hacked.