I have some TV shows I recorded in the 1980s. Ironically, the shows today are far less interesting than the advertisements that were inadvertantly captured.
I've observed the same on some of my old tapes. I think ad's by their nature always try to speak directly the zeitgeist, which creates more interest when viewed from the future than shows themselves.
I have a collection of several dozen Scientific American magazines from the 1970s, they're the same way (although most of the articles are pretty interesting too). The ads are just so dense with information. For example, the Kodak ads will have a full page interview with an engineer describing some arcane chemical process. Now it's usually just a photo and a slogan.