The Bitshares Exchange is decentralised, in the sense that if the exchange entity that backs them, which is currently Open Ledger, becomes insolvent, then any other person or group is able to create a new User Issued Asset that they back with external cryptocurrencies. Centralised exchanges are not able to do this, for example, who can create new "Bitcoins" on Poloniex's servers if Poloniex were to become insolvent? No one. If Open Ledger became insolvent tomorrow and decided to stop accepting withdrawals out to crypto, anyone could come along and replace them on the BitShares network. There is open competition to create User Issued Assets that are solvent, which no other exchange offers.