I dont know why i should not keep my coins in exchanger if i am carefully then i think that nobody cant stole my money from my wallet so i keep my money in exchanger really long time and it is still there in exchanger.
I think you haven't used bitfinex and cryptsy before, because those who think same like you and have stored their bitcoin in those exchanger have already lost all. So if you have to keep bitcoin in exchanger for active trading try to keep bitcoin in multiple exchangers.
I have used them both and fortunately, my anxiety has saved me from losing my coins because everytime I'm done using the exchangers, I would transfer my coins to my Xapo wallet (since they aren't really that much all the time). I'm feel bad for those who have lost their coins to them, but I feel like this should be a good lesson for them, although they paid quite a hefty price for it.