One popular theory among people who don't like Starbucks coffee is that a dark roast tends to make variations in or quality of the coffee beans less important than in the case of a lighter roast.

That said, Starbucks is obviously popular with a lot of people. Personally, I think their coffee and some of their other drinks are fine. I don't always go there (not that I'm a coffeeshop regular) but that's for reasons other than their coffee quality. (I admittedly tend to use darker roasts when I make coffee at home as well.)