The point is that the FitBit does all of the calculation for me, and then I see affirmation in cold, hard numbers that my habits make a difference. I don't want to manually calculate every activity I do and then tally them at the end of the day. That's the whole point of the FitBit in the first place. I want to see that I burned 2,642 calories yesterday, and 2,884 calories today, determine what was different, and continue habits that make a difference.
And yes, if my goal for a 500 calorie deficit, I could eat 2,500 calories on day I burned 3,000, and 2,300 on a day I burn 2,800. What's your point there?
I've logged my calories in (MyFitnessPal) and out (FitBit) for several months now. The line for my expected weight (based on calorie estimates) and actual weight are very similar. Much more than I expected they would be.
To your donut reference, I think what you're getting at is that for most people, food intake is much easier to control and adjust than physical activity. I agree 100%. Eating 100 calories less is much easier and more preferable to me than exerting 100 extra calories of energy. I am a runner, and I believe that running alone is not that great of a weight loss plan. When I run 10 miles I may burn well over 1,000 calories, but I'll be damned if I don't find myself ravenously hungry for quite some time afterwards. I could negate my run in a matter of minutes if I wasn't as aware.
And that's the point - awareness. The FitBit was nice because it held me accountable and gave me data to work with. I wasn't going to pay $100 for something and carry it around all day every day, and then just ignore the data it gave me. I log it every day in Excel. Likewise, the FitBit data alone is pretty useless if I'm oblivious to my intake, so I log intake data every day in Excel. Together, these two numbers keep me focused on my goal and guarantee that I'm progressing toward it.