The assembly was added to the Silverlight suite and as a required reference for RadRichTextBox in the past with future compatibility in mind. However, it was never actually used and was, in fact, empty. Simply removing it from your project should work fine for you.
Please let me know whether this is the case.
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Feedback Portal
and vote to affect the priority of the items