As someone who works out of coffee shops a lot, there is pretty much ZERO appeal for me to use this kind of service, since I view my coffee costs as "rent" and prefer to pay full price plus a tip to be viewed as a good customer. Bringing in an app saying "I'm a cheapskate that doesn't want to pay full price" is not the message I want to give to coffee shop staff.
I do think a shared promotional community between the coffee shops around here (Seattle) would be good, but most are doing pretty well so no one feels any pressure.