All present day exchanges are custodian's. In a pure decentralized exchange the Issuer's handles the role of custodian.  In a traditional scenarios only one custodian handles all the gates for the market place. Hence making the entire market extremely vulnerable to default risk by a single custodian. Like in the case of MTgox. But if Custodian's itself are decentralised then the risk of single custodian defaulting is minimized drastically.