I believe that it is necessary to centralize bitcoins, so that through it do not washed money and do not buy prohibited substances or items. If it is centralized, then a lot of people will believe in it and really start to break the walls of distrust.
Depending on where you live, centralized currency can not solve all the problems involving money laundering and other criminal acts. Fiat currency (cash) is good for those kind of things too. At the end people will always find a way to do their business.
Also large part of community that supports bitcoin, supports it because its decentralized. If you take away that, many would leave and probably everything would go to waste.