Sure they will, coinbase will quite happily turn them back into USD for you.
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.
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.