I've seen comments of that nature for the powering of televisions or laptops and it is a neat idea - while you're burning calories you get to Do Something else. Or while you're Doing Something that usually Isn't Exercise you get exercise. I'm not sure it's entirely practical, but I can see the appeal. But there's something even simpler than that.
When visiting sistaur I used her stationary bike for exercise, but the batteries had run down so I didn't get any indication of speed or distance or calories burned. As I had not much else to do I started pondering that. There I was, burning calories (the whole point of the... exercise, yes) and generating motion which was then... thrown away. Okkay, the front "wheel" is a sort of fan so there is some self-cooling involved. But it seems that a small (even tiny) generator, a regulator, and maybe a rechargeable battery, would be the right way to go. Gadget not displaying? Throw some calories at it, it'll come right up! It's not as if you were going to be conserving those calories. They might as well be made useful even if only in a small way.