This. As much as it may work right now, because basically no one is doing it, as soon as it becomes a popular method, it seem to me it will stop working pretty quickly. Everyone doing it will become equivalent to a Jehova's Witness knocking on your door.
That said, I imagine I'd be more okay with it if every time I said yes I got a free coffee.
Aside: I wonder how different reactions would be in different countries/cultures.