So if you had already done a legitimate trade it's gone? "Sorry you made some money there, but someone else got affect by something else, so we've undone that". So much for "There are no chargebacks on BitCoin"
Take the flash-crash last year. Yes, if you were a lucky one who bought GE at $3/share then hell yes you wanted that trade to count.
But to have an exchange it takes everybody acting in the interest of the group as well as themselves. The value to that over purely selfish motivations is that it creates a liquid market which benefits everybody involved.
Part of this is the acceptance of situations like this. In cases of attack or software defect, the only real viable option is to rollback. The only people this hurts are the 1% who tried to profiteer on the situation. Not rolling back would harm the other 99%. It's an easy call.
The only real tragedy would be if they cannot rollback accurately. You'd think this wouldn't be possible, but you never know... It seems as tho this site has had a known CSRF bugs for a while. This is not a hard thing to fix. It doesn't shine well upon their competence.
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.
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.
Well this is trading through an intermediary, you can't really expect it to work that way otherwise something like this would have a lot more incentive for wrongdoers.
If you really want to have no chargebacks you have to go the harder route of one on one trading, with everything that comes along with that.