Even if Bitcoin balances can be made bullet proof, the Achilles heal is that it is not a legal tender, and that there are no market fundamentals to contribute to its exchange rate. My 660 Trillion Zimbabwe dollars, sitting here on the desk, are solidly in my possession. So what? P=0 means they have zero use value other than as a conservation piece. Interestingly, when I show them to people and they handle them (Produced by the British Bank Note Company that makes currency for a lot of countries) they frequently falsely conclude that the are phony. They have a hard time reconciling "real" with "worthless"
Whether or not Gox slid into fractional reserve banking is interesting, since if they did (I doubt it) they were basically betting against their depositors. To do that they would have had to taken Bitcoin balances as they came in, sold them for hard currency, wanted for Bitcoin to fall, buy it back, and pocket the difference, at the same time being able to honor withdrawals so there was not a bank run. The way this ended suggests that either something else happened, or they "bet the farm" and shifted virtually all of the Bitcoin deposits at a hard currency Bitcoin price below what it is worth now. The got caught "short" bigtime like in a margin call for stock bought on the margin.