Bitcoin has absurd volatility.
LOTS of time it varied 5% in 30 minutes. There are days with 300% or more variation.
Circuit breakers would only cause people to move to other exchanges.
There are other security measures that can be done, that are not circuit breakers (like requiring confirmation, warning the operator to watch what is happening, etc...)
Nothing could have stopped the selloff except an automated circuit breaker. MagicalTux has to sleep sometime, so operator warnings won't work, confirmation only works if the confirmation is secure, since the accounts were hacked we could assume the confirmation mechanism would have been broken too.
Maybe my numbers were too conservative, cut off the exchange until human intervention in the event of a 50% fall in value in less than half an hour. I think that's quite unlikely, and as far as I can tell from a very quick skim of prices hasn't happened (and gets less likely to happen the bigger the bitcoin economy becomes).