Locals love Starbucks because of the brand name/marketing, the comfortable chairs/couches, and the sugary drinks like the dulce de leche latte. TONS of people order those 1000+ calorie drinks. Starbucks doesn't need to have good coffee because I'd wager less than 10% of customers order straight coffee. Argentines also tend to put a lot of sugar in their coffee anyways, so quality is less important.
It all comes down to personal taste, but in my opinion Starbucks is no better or worse than the big Argentine coffee chains. Cafe Martinez, Havanna and Starbucks are all mediocre. It just depends which brand of mediocrity you prefer. Starbucks is just more expensive because of the brand name and yankee fame.
Oh, and Starbucks will serve you coffee in a normal ceramic cup. You just have to ask for it.grl