The loss rate is extremely high; I don't think people would be willing to pay an extra 20% spread on their exchange to cover it.
Much higher levels of isolation (such as running the transaction engine in some kind of HSM) would help, but I suspect that anyone smart enough and cautious enough to do it properly is too smart and cautious to go anywhere near the prospect of running a bitcoin exchange.
Much higher levels of isolation (such as running the transaction engine in some kind of HSM) would help, but I suspect that anyone smart enough and cautious enough to do it properly is too smart and cautious to go anywhere near the prospect of running a bitcoin exchange.