I usually do not keep more than 10% of my funds on any exchange. You cant be always sure about the security of your funds so having a secure cold storage is what I trust and use to store my bitcoins. There is 2-factor authentication enabled but still it does not guaranty the full proof safety and it has nothing to do with the hacking of the website network. The best way to trust on self and maintain good and secure cold storage.
That is also I a doing, not all bitcoin is keep on exchanges but of course as a trader we have unds on standy at different exchanges, so that when there is opportunity for some coins,we can buy fast and trade. Just a minimal fund for trading and also a fund that I can afford to lose.
I had some bad experience when I kept nothing on exchange and I lost some good opportunities so you have to keep something if you dont want to miss good opportunities. Yes there is risk associated with it but trading is all about risk and if one is not brave enough to take this basic risk then he should not trade further imo.