I don't think traders, specially noobs are very complacent on trusting web wallets like exchanges. But throughout the history of Bitcoin, we all know that is not the best practice as anytime those exchanges can be hacked or exit scams and their money will simply disappear in an instant. Perhaps those traders will need to get that experienced first hand, before they really stop or at least not putting huge amount of money in an exchange, expensive lessons, but what can we do? We have time and again, tell them that if you don't have the keys, then it's not your coins, but they didn't heed until they got their first hand experience losing big money here.
Using up service do have corresponding risk and we can lessen it up if we do know how to follow those safety tips from time to time.