I'll take a stab at a response. Fellow developers, please correct me if my understanding is not correct.
I think that you have to go with the Engine solution because I believe that both the Silverlight and WPF solutions require ArcGIS Server, i.e. they were not developed (as of yet) to support disconnected situations.
FWIW, I believe that in 10.1 there is the new ArcGIS Runtime which provides an alternative to Engine.
For Engine, Desktop, and Server it is possible to customize the system using low level (fined grained) ArcObject APIs.
Is there any similar provision for ArcGIS Runtime?
Essentially, do you have an extension mechanism which runs in disconnected environments which allows a non-ESRI developer to access ArcObjects?