I think it's much more of a "I already write C#, so now I can write apps for other platforms without needing to learn their language!"

That leads to further evangelism at conferences, more talk about how C# is great and the platform is excellent, etc. I think they hope to persuade entire dev shops to switch over, rather than single devs, in a very long-term sort of way -- even if it's just a small percentage that change, the press is potentially worth it. MS has an image problem, and this would help mobilize their core to speak to more of the good parts.