As we all know exchanges can get hacked, caн be down for a long time, can held transactions and so on, so obviously it is not a good idea to keep your coins in them at all times. But then comes the transaction prices, if we move the coins always, we pay extra for the transactions we make. Perhaps the decentralized exchanges where you can directly log in with your wallet will help solve this, as you have the control over your wallet and not the exchanges.
If you trade for a living, that means almost daily, moving coins in and out of exchanges makes no sense.
So you will have to store a certain amount of coins on the exchanges of your choice.
Therefore I only store small amount of coins, compared to what I have in cold storage, on exchanges. Furthermore I also withdraw regularly some of the profits.
So if something should happen with one of those exchanges, the loss wouldn't hurt that much. That's the risk we have to take at the moment.
Decentralized exchanges will need much more time to be competitive with centralized exchanges like Binance or Bitfinex in terms of number of users, liquidity and ease of use.
But I hope one day we will get rid off all those centralized platforms.