In principle, it doesn't affect that goal. An exchange doesn't control monetary policy, i.e., it doesn't set interest rates or increase the quantity of currency.
Of course, an exchange can affect the value of the products it is trading. Since Mt. Gox is the biggest bitcoin exchange, if a vulnerability is found then people might stop trading there, decreasing the liquidity and consequently the value of bitcoin. But this doesn't contradict the goal you mention.
As an analogy, if company ACME only trades on Nasdaq and there's a system problem with Nasdaq, some people will be scrambling to get rid of their ACME stocks so ACME's price will go down. That doesn't mean that the exchange controls the price of ACME.
But if MtGox's technical failure effectively took USD$120M of bitcoins offline, then MtGox has reduced the supply of BTC for some period of time, no? I understand that exchanges are secondary markets, but they seem to be contrary to the philosophy of no centralized control over supply/demand. I always understood bitcoin to be a per-transaction, ad hoc currency – not an exchange-traded commodity. Of course, people are free to do whatever they want with their stuff...
Edit: I realized after posting that I called bitcoin a "currency" above. I realize that it's a controversial position, but my feeling is that, as a medium of exchange, it qualifies a currency.
Of course, an exchange can affect the value of the products it is trading. Since Mt. Gox is the biggest bitcoin exchange, if a vulnerability is found then people might stop trading there, decreasing the liquidity and consequently the value of bitcoin. But this doesn't contradict the goal you mention.
As an analogy, if company ACME only trades on Nasdaq and there's a system problem with Nasdaq, some people will be scrambling to get rid of their ACME stocks so ACME's price will go down. That doesn't mean that the exchange controls the price of ACME.