From the btc be stolen of bitfinex , it should be a lesson for the most, in the past mr gox has scammed a lot users here, soo we do learn with mistakes and no one should let all the coins at one exchange, doing that is taking a huge risk to loose those.
There are certain aspects as to why people will never learn from past happenings. First aspect is that they constantly focus on the security of the exchange itself, which of course is also very important, but they don't focus on the operators of the exchange in question to be the thief here. If the security looks all right for people, then they put their full trust in that exchange. Another aspect is that people are too lazy to constantly cash out and deposit coins. They want to sell or buy at the time some action in the market appears. That's only possible if their coins are constantly stored there.
