from what i understand of coinbase... the goal of coinbase is more to be a merchant/customer service, like the "paypal of bitcoins" ... to make it easier for companies to accept bitcoins as a payment without having to actually worry about any kind of risk that many people associate with the word bitcoin because they (the merchant) are just receiving USD/currency, not BTC, as coinbase automatically does the transaction of BTC->Currency for them.

coinbase isn't really meant to be a BTC trading platform (MtGox). They aren't directly competing against MtGox because they are providing a different kind of service, one that is geared more towards merchants and being about as user (customer) friendly as paypal.