In the beginning I thought demo reels were a waste of time-- both as a producer and a viewer. Showing just the best bits, there's so much more that determines whether a video is good-- story, editing, and performances. As a photographer, I would post links to the top 20 images from a single wedding, so that couples could get a real sense of that to expect.
But lately I've decided that they do have some purpose. Yes, there's showing your very best bits quickly and concisely, but more helpful I think is to show "boring" things in interesting ways.
Unless your interested in the subject matter, interviews are pretty boring. But they're also the centerpiece of many commercial videos and documentaries. They're vital to Kickstarter/Indiegogo/MicroVentrues campaigns, tutorials, and company intros. If you're a business owner, entrepreneur, or artists, you should be paying close attention to the quality of the interview technique.
We created our Interview Video Reel so that prospective clients could quickly see what our work, and learn a little bit about what makes an interview stand out from the technical standpoint.