My personal test for documentaries about entertainers is this: If the amount of time spent on other celebrities comments about the subject's "importance" vs. the amount of time showing actual clips of the subject in action is the same or greater, the documentary fails the test, and I go for rewind and eject. This documentary failed the test within the first ten minutes.
I suspect that documentary producers do this to save money they would otherwise have to pay in royalties for the clips. Otherwise, I can't imagine what compels them to use such a format.
In the case of Jack Paar, if his talents are worth learning about, and, by the way, I believe that they are, let's just see his performances unbroken and uninterrupted. The only additions should be those that put his work in historical context, or that flesh out his personality and life story. In any of these cases, the additions should be brief and inserted judiciously.