Ken Burns had Morgan Freeman recite the words of Frederick Douglas.  He didn't ask Mr. Freeman to comment on Douglas' historical significance.  He let Douglas speak for himself, sparingly appending the thoughts of some actual historians.  

I greatly respect the work of many of the celebrities who comment in the CBC series in their respective fields.  But would not expect them to give me a fully  rounded view of history any more than I would expect them to fix my car or wire my house.