Leaving bitcoins and other crypto coins at an exchange is a large security risk in my opinion. There have been big exchanges being hacked in the past, leaving investors with a losses because recovering the coins is very hard. It might be easier to not always withdraw coins to your wallet, and you can also save the transaction fee every time, but you could lose everything like that. Better be safe even if there never was an issue. Receiving interest on your coins is nice, but is that profitable? I wouldn't expect an exchange to pay high interest, because it will eat away their profits. Also the exchange doesn't really need your coins if it only matches buyers and seller. In case the exchange is also lending out coins it might be different, then there is more risk involved. What if all the borrowers default at the same time and leave the company hanging? In case of bankruptcy I wouldn't expect you to get all your coins back either. If you only trade a few times per month, do the extra work and save your coins in a seperate wallet.