There's a lot of justify your platform going on at the moment. I see Mike Taulty has weighed in on the subject.
The truth is HTML is still HTML and Flash and Silverlight are not just about streaming video.
Then there's the whole question of running disconnected. Page storage is all well and good but what happens when *shudder* you have to move to another page? What about device integration?
The truth is Silverlight (and Flash) presents real value to application developers but is a bit difficult to quantify if you haven't felt the pain of trying to develop large HTML apps.
But that's just me.