the exchange as an exchange was not compromised. at most, mt. gox as a broker, or an individual user account, was compromised (putting aside the leak of information, which hasn't been cited as a reason for a 'rollback'). the analogies to the 'flash crash' in the us stock markets is inapposite.
I normally respect your views on things quite a bit, so I'm willing to consider that I'm on the wrong side of this debate. Just to be clear, I don't have any trades that would be cancelled, I assume you don't either?
thanks for the kind words. i agree it's a complicated question, and reflection is useful.
to answer your question, you're correct: i don't have any trades that would be canceled. indeed, i don't even have a mt. gox account, having not trusted them for a long time (though i don't mean that specifically to impugn anything about their service - i simply lack trust in it). as usual, i'm not writing to the forum out of a narrow financial self-interest.
the best way, in my view, to understand the problem is to recognise that the part of mt. gox that implements a currency or commodity exchange was apparently not hacked or compromised in any way. if it had been, then trying to break executed trades would clearly be appropriate. but all that's reportedly happened is that mt. gox has identified what it believes is a theft, or a collection of thefts. to identify, after the fact, a theft that moved a market provides little reason to break a trade. as analogies, consider: (1) if the stolen amount had been only 500 btc, you'd never consider taking the money back from someone on the other side of a trade from the thief, and (2) if someone had stolen us dollars and used them to buy bitcoins on mt. gox, aiming to transfer them out before being detected, similarly mt. gox would probably not have thought to break any trades merely because the price of bitcoins temporarily doubled.
thus, if what they propose is done only in response to events on one side of the market, and only in response to arbitrarily selected events, it appears unprincipled and of little value to anyone. that said, i don't personally have a strong opinion about it; largely it's a contractual matter between mt. gox and its customers, and it threatens bitcoin as a technology only to the extent it undermines confidence in what is unfortunately a very concentrated market for currency exchanges. i think that on balance it would be a bad idea, systemically speaking.
I have a more practical question. Is Mt. Gox likely to have the capital to eat the bad trades if they didn't do a rollback? I understand some ~260k were moved at $0.01 Beyond the trades on the compromised account that they'd have to eat, I assume everyone who sold into the market as it crashed would want a refund based on the idea that it was MG's negligence that caused the move. Let's make it overly simple and say they have to come up with 500,000 BTC (todays volume-avg. volume) to make good.
So simplistically 500,000*$17.50= $8.75MM. I've never paid too much attention to how much money they've been making on trading volume, but it'd surprise me if they have that kind of liquid assets. If they couldn't cover all of the wrong side of all of the trades it seems they'd end up insolvent and potentially be unable to pay out even some regular depositors. That hardly sounds like an optimal solution.
reports were that it was making $70,000 per day, but not presumably for very long. i doubt it would have the capacity to insure all user accounts or even this one large one, and it's not clear that it would have a responsibility to do so. but that's a separate question from whether they break trades made in good faith.