It's more convenient to swipe to open the card directly from the lock screen instead of unlocking then going to the Starbucks app then waiting for it to load. True, it saves mere seconds, but convenience is convenience all the same.
Apologies if this is a dumb question but as I don't have Passbook yet I don't know. Wouldn't you still have to swipe thru your list of Passbook cards though to find it? Whereas with the standalone app I know exactly where to find it.