Exchanges are meant for trading the coins, it is not for storing the coins. I hope if you show interest towards trading then you can move your fund from wallets to exchanges. If you don't have any experience better move them to your wallets only. For everything, there is risk involved in it.
If someone has a cold storage and rarely trades I see the point not exposing your token to unneccessary risks. But if someone has no cold storage just private keys and trades often it can make sense to leve a fraction of your token on exchanges, especially if they are secure ones. Cobinhood has 97% in cold storage. If you trade a lot and have to transfer your tokens constantly to the exchange and back to your wallet, dont you think there is some vulnerability too? Someone could spoof your key, you could make a mistake and send an token to the wrong wallet (Erc-20 token to Bitcoin Wallet). I think a lot depends on your personal preferences and your skills.
Ideally everyone had a cold storage at home of course but most do not from what I hear so far.