It's more that you cannot physically shutter a decentralized platform down in the same manner you could a centralized one.
When GLBSE, BTCT.co and Bitfunder went dark, alot of people lost money as a result, Asset issuers and shareholders then had to scramble to transfer shares. Some made this process a very smooth experience, others made this complicated and unwieldy. You cannot get goxxed by incompetence of a 'PHP can do anything' type of guy that bit off way more than they can chew, for example with a securely designed protocol because your money is in your hands and you know whether you can trust yourself. The amount of outright scams, hacks and losses over the past years from storing your private keys online is just shocking, totally sick. these decentralized exchanges can allow the owner to take their own custody of keys. The brilliance of bitcoin was the protocol allowed you to be your own bank, now you can be your own exchange.

I'm a fan of the concept of these next-gen layers that will decentralize what was previously only available with caveat of a single points of failure and required blindly trusting the competence & integrity of the operators. For instance, trustless asset exchange, decentralized betting markets & derivatives. I don't think at all that they will entirely replace centralized exchanges at all because there is many features which they simply cannot compete with. they are not suitable for 'high frequency' style trades- by comparison they are cumbersome, and f course due to the fact there is no central entity which governs and filters listings, no listing fees, no verification there will be an influx of scams. Asset issuers are able to give a description with a signed message for instance and a link to a web-of trust or other such identity mechanism, it will really be honest people remaining honest and having to do own due-diligence for any potential investor for the time being until some serious effort goes into working on a 'cheat-resistant' rating and trust authentication scheme . even high-profile centralized exchanges have definately not been free from scams either, see for instance labcoin. in which the asset was approved for listing on btct, raising multi-million $ worth in bitcoin and exchange allowing withdrawal of that total amount shortly after, all with not so much as a simple identity document. this was after many, many concerns were raised and red-flags all over the place.

We are still in a very early phase of these types of services. I have looked at several implementations including, coinprism (predictous' colored coins), SFETS, Nxt, MSC , CounterParty) My personal favorite and one I believe has the brightest future ahead is counterwallet from the counterparty XCP community. You can test some basic asset creation function on an alpha web-wallet here. https://testnet.counterwallet.co/# example of assets. http://www.blockscan.com/assetInfo.aspx?q=MPTSTOCK