It's not really safe to keep those Bitcoins in an exchange, that has been said over and over but there are people who keeps them there and it just tempting for the site owners if they see a lot of zeros stored in their. Not blaming anyone here but lets not give them the reason to run away with our coins. Even the good guys will turn their back when they see an opportunity to earn easy money and remain anonymous.
You can't blame them. How can you do such trades if you will regularly pullouting your funds. An active trader needs to put all his trading funds in an exchange so that they won't be late when there is a price rally.
For storage that's not an option but for a regular traders it's a must.