The two mentioned assemblies were introduced in Q3 2013 (2013.3.1016). Previous versions of the Telerik UI for Silverlight suite do not include the two dlls and RadRichTextBox do not need a reference to them in order to run.
If your application uses a recent version of the suite or you are upgrading to such version (one after the mentioned above Q3 2013), I would advise you to add a reference to the two assemblies. There are members of them that are needed when you run the control and we plan on introducing other changes that would require the two assemblies in the future releases too.
I hope this helps.
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