It's also great if you just want to stream content. Streaming WMV to Silverlight needs much less of a backend than F4V to Flash, not to mention that Flash eats resources like a hungry kitten on the frontend.
Now, I'm not saying that it's any good, 'cause it isn't, but it's certainly compelling for sites that stream a lot of content. They can do it with a fifth of the servers they need for Flash (apparently, I'm getting my figures from our public broadcasting system which went Silverlight around the last Olympics and never went back. They do have the option of streaming "as is" WMVs though, so I can still access them from my Linux box).