From a business standpoint, it looks like a loser, since I don't think many people go out to eat or drink hoping to discuss political/social issues with their server. I sure don't.
It's also tough to see how this can accomplish anything real positive about race relations; there's unlikely to be much give-and-take in the discussions – on the rare occasions I've been in a Starbucks, the clientele seems heavily skewed to upper middle-class white twenty- and thirty-somethings whose thoughts on racial issues seem unlikely to differ much from the gender-studies and art history majors who will be preaching to them. Of course, that might well be the point – it could be that this is simply an opportunity for the company to preen in front of its core demographic. Ben & Jerry's has done well with this approach.
There is another possible benefit – some folks who have never visited a Starbucks might be tempted to go there just to have an opportunity to tell the barista to STFU. Hey, companies lay out lots of marketing bucks to get non-customers to try their products, this could be a relatively cheap way to accomplish the goal.