I'm not so sure about the argument that most people would forgo the $4 cup of Starbucks, given a free alternative. Every office that I've ever worked in provided free coffee, and half of the office would still regularly pick up coffee from the nearest Starbucks daily. I think there's something to be said for

1) Crafting the "image" of superiority that people get from purchasing Starbucks (having that nice branded Starbucks cup, rather than the crappy styrofoam one your office provides, allowing people to order ridiculously tailored drinks -- soy half caff with a dollop of creme...etc) and

2) Providing an ecosystem to encourage the purchase over free alternatives (I've noticed that I'll stop at Starbucks for a snack because I'm hungry in the morning, and pick up coffee/tea as well, just because I'm there)