Does anyone see value in this at all? I don't see why you would want to put money into an exchange instead of a bank account. Seems rather dangerous to me....
Instant trades between USD and BTC without involving the slow banking system. This is the only real reason to 'store USD' at Coinbase for any period of time.
I'd say that your average Bitcoin user doesn't need this feature, but it does expand the potential of Coinbase as a trading platform (store some money there and trade when you think the prices are right), instead of just a place to buy and sell Bitcoin.