There's one moment when Mac users almost universally cringe in fear: finding a dreaded streaming Windows Media file on the Internet. Windows Media Player for the Mac stopped at version 9, two releases and three years behind the current Windows edition, and sometimes the old version just won't play nice with streaming videos. Sure, Microsoft has provided Flip4Mac to give Mac users a bit more Windows Media support, but even Flip4Mac can't handle everything, and Microsoft's kindly gesture is more of a band-aid than a permanent solution. And don't even get me started about the lack of Windows Media DRM support.
The bottom line is this: Silverlight isn't going to be a newer, standalone Windows Media Player, and I doubt we'll ever see another one of those. What it will do is provide much better support for streaming video on the Internet, especially Windows Media 11 files. Since a lot of the video we watch today is on the Internet anyway, and since we'll probably be watching more and more video online in the future, Silverlight seems like a good deal for Mac users. There's no word yet on DRM support, and I would be surprised if they included it, but a streaming Windows Media plugin with DRM support would be an even bigger deal for Mac users.
Also worth noting is the comment buried in the MacBU post that the .NET Framework core components are getting ported to the Mac. Depending on exactly what is getting ported and to what extent, this could be another big deal. It will mean more Mac interoperability, more opportunities for developers, and the suggestion that Microsoft is taking the Mac a bit more seriously.
I think that even if Silverlight turns out to be complete crap, the ported .NET Framework and the increased attention to Mac users will be beneficial in the long run. I don't actually think Silverlight will be complete crap though—although it adds a bunch of unnecessary stuff on top of the basic streaming video functionality, having an official way to play Windows Media files online will be a big plus for Mac users.