If you have bitcoins on Mt.Gox, sure, you can get USD for them elsewhere.
The issue I was raising was that if you have USD on Mt.Gox and people don't think USD will be able to be extracted from Mt.Gox, you will have trouble converting your USD to bitcoin to get it off Mt.Gox, because people who have bitcoin on Mt.Gox will be reluctant to convert it to USD on Mt.Gox.
> That's true, but I'm unclear why you'd store USD for any length of time on mtgox anyway.
The same reason you'd keep any currency in an account tied to a particular marketplace: liquidity that allows you to trade without the delay of getting money into the marketplace from somewhere else.
It takes a long time to transfer money into MtGox and with the wild price fluctuations it might be prudent to wait for a better price instead of buying at the current market price the second the transfer clears.
The issue I was raising was that if you have USD on Mt.Gox and people don't think USD will be able to be extracted from Mt.Gox, you will have trouble converting your USD to bitcoin to get it off Mt.Gox, because people who have bitcoin on Mt.Gox will be reluctant to convert it to USD on Mt.Gox.