I have just about completed a WPF application that could not really work as a Silverlight application because I need to communicate with windows and COM components.

 

The emphasis is on Silverlight because the web works best for a lot of applications, but there is still a requirement for desktop applications and thanks to Silverlight, WPF has gotten a lot of controls that would otherwise have been left to third parties. Look at the WPF ribbon for example, that has taken over a year, and it still has not been completed. If it'd been a silverlight control it would have been completed several months ago.