...I'm really not sure why people think that centralized exchanges are vital to the bitcoin economy. Trading p2p is much more efficient and people in china are already starting to do it.
...
Because a big part of the current user base of Bitcoin consists of traders. They need exchanges with an orderbook and preferably also
features like margin trading. OTC or P2P trading may be fine for the casual user, who needs to buy or sell BTC once in a while.
However, it is absolutely impractical for traders, who do multiple trades a day. Especially the guys, who are doing arbitrage or
use scalping-based trading strategies.