Answer
The ArcGIS Server Web ADF Runtime for the Microsoft .Net Framework is only needed if a distributed install with the Web ADF is being used on another computer.
The following is a stand-alone setup for Web ADF Runtime:
ArcGIS Server Web ADF Runtime for the Microsoft .Net Framework
ArcGISServerWebADFRuntime92sp*.msp 6 MB
This is also a feature of ArcGIS Server .NET. The Service Pack listed for ArcGIS Server Web ADF Runtime for the Microsoft .Net Framework is for the clients of ArcGIS Server .NET that only need the runtime to use an application developed by a ArcGIS Server .NET developer.
If both ArcGIS Server and the application development is installed on one computer, install the following:
ArcGIS Server for the Microsoft .Net Framework
ArcGISServerDotNet92sp*.msp 88 MB