I think akerro is asking about the motivation for Microsoft's contributions to OSS, rather than whether they benefit from it (clearly they do). My impression,

I think akerro is asking about the motivation for Microsoft's contributions to OSS, rather than whether they benefit from it (clearly they do). My impression, having spoken to people from Microsoft involved in OSS, is that they are pretty much entirely driven by what will make them the most profit. Sometimes that means making software "open source" and accepting contributions from the public (free labour!), other times that means keeping their software proprietary. They don't seem to care about ethics, or software freedom. It's a shame, but Microsoft aren't the only ones with that mentality. Big businesses are, more often than not, primarily motivated by profit.