1. That DRM systems don't work 2. That DRM systems are bad for society 3. That DRM systems are bad for business 4. That DRM systems are bad for artists 5. That DRM is a bad business-move for MSFT
Whether you like Microsoft or not, you've got to recognize that they haven't gotten to be the richest company in the world through stupidity. DRM is long-term stupid. It's technology designed to make technological products less useful ("Where do you want to frustrate your customers from doing today?"), and it doesn't work as long as there's one determined attacker in the audience.
The DRM moment has been left behind by science. Publishers were looking for pay-per-use and perfect price discrimination; DRM promised it to them. But DRM was backed by bad science. As long as we live in a world where we can still talk to our friends and still tinker with our tools, DRM is doomed to failure. And when it fails at its primary purpose, it succeeds only at driving potential customers to other sources.
In the short term, DRM may help facilitate lock-in to a particular manufacturer's products. Once you buy a few Microsoft media player tracks, it's easier to keep buying Microsoft. But as the format gets less useful, and the media player's requirements become more restrictive, OGG looks more attractive. Sure, it'll take some effort to get your existing tracks back (you might have to convert, re-purchase, or find clear versions on the Darket), but once you see the gains in flexibility, you're unlikely to look back. Customers jump ship from DRM, with best customers first over the edge. If Microsoft as technology company doesn't see that, it's just ceding its leadership to someone who does.