The killer app for a Bitcoin exchange would be to give customers an account number they can use for ACH push transactions and direct deposit.
This would dramatically increase the risk of fraud. Banks spend billions of dollars on fraud prevention and detection and they still can't totally eliminate it but at least their deposits are insured. By comparison, Bitcoin exchanges are tiny and can only mitigate some risks by avoiding them altogether. Their deposits aren't insured and they don't have unlimited reserves to cover losses - as the Tradehill/Dwolla debacle showed.