Why We Have Centralized Exchanges
While the design of cryptocurrency is to use the block chain to track transactions, and yes the block chain itself is more secure and simplifies the tracking of currency investment not all currencies are Fiat making fully decentralized exchanges less liquid. These exchanges lack some of the tools centralized exchanges have developed for advanced analytics , tax documentation and other advanced investment functionality.
However these exchanges are off the blockchain and have been vulnerable to hacking. The transactions are also not recorded in the blockchain which means there is no audit trail.
The Evolution
As the the cryptocurrency space evolves many companies are focused on bridging the gap between the traditional investment market and the cryptocurrency market. Legolas Exchange is a semi centralized exchange which will provide investors with the same tools of a centralized exchange but utilizing the block chain to provide security and speed. To see how they achieve this checkout their recent Medium post on decentralized exchanges vs centralized exchanges.
Their ICO begins on the 1st of February.
As more people begin to invest in Cryptocurrency there is an obvious need for exchanges catering to all investors.