I've spent hours in a few different Starbucks in the south bay before. My observation has been that most people either get their order and leave, or stick around for an hour+. The number of people who sit down just long enough to eat a muffin or something is pretty small.
Why is that argument stronger? You think Starbucks hasn't run the numbers/experiments to test whether a warm body on a laptop is better than the same body ducking in and out? I would bet money they have done a frightening amount of analysis on this question.
Also, to your specific point - when people choose restaurants they often go where there's a crowd (the food must be good here), not to an empty restaurant. The situation is a little different because Starbucks is so pervasive, but don't discount the customer draw of a bustling store full of people.
Over the years just kind of casually observing the placement and layout of the stores, it seems that they are maximized to be easy to access by the morning rush crowds as well as the lunch and after work rushes. Also, a pronounced increase in drive-through locations.
I could imagine that the majority of a location's revenue comes from walk-in/out business during peak times, and the tables could stay full with little rotation all day without appreciable impact on revenue, while solidifying the chain as a go-to "third place", especially for the business class.
i'm guessing there are very few people who go to starbucks, see the seating area is full, and leave. If they're already there, they're going to buy something. If it is busy they might order and then go to a nearby park or something, but once they're at the door they're pretty much a guaranteed sale.
