I think users should not leave BTC at exchanges. In such cases, you will have control over both coins in case a hard fork occurs. You also minimize the risk of losing money if exchanges get hacked.
I highly doubt if any of the top exchanges would con you for the split coins. Majority of the people who have their holdings at exchanges are traders and they need those there. For everyone else, who's NOT tech-savvy and not acquainted to understand the security of their personal computers/hand held devices, its better that they hold their coins at an reputable exchange than keep it on their device. The probability of your device getting compromised is much higher than an exchange. Plus exchanges today hardly keep many coins online. For peace of mind, you may diversify any risk by splitting your holdings into multiple top exchanges.
Its needless to say, people w/ larger holdings already keep their coins in hardware wallets or in cold storage.