Because Microsoft's monopolies are hurting competition and the evolution of our practices. We certainly don't need them pushing a new search engine through MSN Messenger or Windows updates.
Perhaps, but is Microsoft, a convicted monopoly abuser, the right challenger? Microsoft has already a powerful grip on the whole industry (why do you think multi-processors and 64-bit computing took that long to catch?) for us to feel too comfortable with them gaining more power.
Google has a monopoly on search, but Microsoft has a couple at least as valuable as Google's.
Competition between giants is a good thing. And a sense of urgency in the marketplace is good for third party companies, as it places pressure to buy rather than build.