I agree that can work pretty good, but you really have to know what's going on with everything and constantly update it to account for any changes made in the system (ie the whole car).
Ultimately, as stated already, all this is doing for you really is giving you a delta to show where you're getting the best efficiency. For example, your calculations may show you're getting 28mpg at 5th gear cruising and 12mpg when passing, but you could very easily be getting 22mpg at cruise and 6mpg when passing. So, now you'd know you were getting better mileage at cruise than in passing.
Having knocked the idea, I wouldn't mind doing something similar. However, given that I can check my average mileage upon fueling, I can just look at a wide or narrowband O2 to determine when I'm running leanest and consequently getting the best mileage. Agreed?