Yes. The vast majority of payments I see in Starbucks in the US are via credit cards (or debit cards). With modern PoS systems, paying via card is faster and more convenient than cash, not to mention other benefits (rewards on cards, being able to better track spending)
I used to work at Starbucks (albeit going on 5 years ago) and although I don't have the exact statistics or a perfect memory, it was not a "vast majority". If I had to guess, I'd say probably something like 2/3. Maybe the ratio increases if you count Starbucks gift cards.
Either way you look at it, Square's transaction volume surges with this deal. Good on both parties.
