RMR is a useful benchmark for a minimum amount to eat in relation to a person's size. If you want to do it on less than that work with a doctor or dietician. Two other cross checks on weight loss are max -2 lbs/week or -1% of body weight.

"Each pound lost is 3500 calories under consumed for a week" wrong

"Each pound lost is either 3500 calories from stored-released-burned fat (454 grams), regardless of time, OR it is 454 grams of anything else being removed from the body or converted and then removed from the body (water, water+glycogen, amino's, etc).

Lets say your BMR is 2000 and you decide to fast for 72 hrs AND you decide to walk for an extra 1500 calories a day, and you lose 5lbs of scale weight. What did you lose?

Well, I don't know, in order to find out, drink water and eat just carbs until your carb loaded like you were when you started and THEN weigh yourself. Bet it will be 1-2lbs of fat lost.

Protein intake and nutrient density are a more useful benchmark, as bodies are not looking for calories per se, but nutrition. I paid absolutely no attention to RMR, made an educated guess as to how much protein I needed to maintain muscle mass, and generally tried to stay under 1500. Sometimes the scale seemed to refuse to budge and I'd go really low carb, or give up my wine and chocolate, but on reflection I have to say I was probably losing weight as fast as humanly possible and extra strictness didn't give me much return on investment.

RMR may make more sense to pay attention to as you get closer to your goals. As BMI starts to look less like a ridiculous made up number and more like a reasonable target, or even a target you might have to put on muscle achieve, the body seems less happy about converting bodyfat into fuel. Calories become a lot more important then.

RMR is the minimum amount of "raw material" needed - I believe - for the body to adequately repair and grow. When you eat below RMR for too long, repair of some things can lag behind. This would explain why many chronic dieters appear aged and have worse hair, teeth, nails, and skin (grayish?).

I believe this is why when ED recoverers start off, they have to eat 4000+ calories a day and don't gain much fat. The body uses a lot of this food for things like muscle and degraded tissue, trying to "catch up" on what it's been unable to do for too long.

Eating below RMR does way more harm than good if weight loss is your goal. Weight loss should be slow and steady, after all. Food is medicine, so constant restriction can't be healthy IMO.

Your body would go into starvation mode after a few days, slowing down metabolism to preserve energy. He doesn't know that there's actually plenty of food available and try's to retain as much energy (=fat) as possible to survive. This is not what you want to lose weight. :)