That's an interesting point--I have a copy of Larry's radio interview that I cannot bring myself to listen to. However, I've watched his last on camera interview many times. I can't explain why seeing and hearing him would make that much of a difference--perhaps it's because I could get a better sense of his humor and expressions by seeing him.
Ed Bernds mentioned how difficult it was to see Larry at various personal appearances, and from his point of view, that's understandable. But for me--the fact that he was doing as well as he was (and this was in the 1970s!) surprised me.
But speaking as a fan who will buy almost anything book or film related to the Stooges, BeAStooge's right. No real insights, but the footage alone is a good reason to buy it.
(Since I'm over 20 days late in reading this, you may have already purchased it!)