And they get more than just code, and people who write code. They get a little street cred back in the hacker community for picking up a YC startup. They get people who have vision, which MSFT appears to be in need of these days. It could be the beginning of a new era for them.
Also, Xobni has had a number of high quality engineers (probably much better than what you'd typically find slaving away in the trenches at MSFT) working on their plugin for a lot more than a few months. I'm guessing the programming cost would be a lot higher than you might think.