People will go to sell at the highest price possible which is what Mt Gox has been doing, so you have camp bx do the same thing, btc-e, etc, the amount of people needed to ddos 4-5 sites compared to 1 main exchange, less chance of panic selling, and a faster growing currency.
I think that people are DDoSing in order to make profit and increase the price of btc.
That's not because you can't access mtgox for 1 day that btc will loose is value. But you can't cancel yours orders. And when the price increased by 50 usd during the DDoS, you got it sold 50 usd less than the market price when the servers are up again.
And the markets for bitcoins is not centralized. It will not be possible to constraint every exchanges to share the same database / prices for bitcoins. That's not gonna happen, even if 2 or 3 accept it, the other ones won't.