the first comment comes from the issue with Ketosis. With ketosis you take fat and convert it to a form of chemical that mainly certain organs require for energy as they cannot directly burn fat. This chemical form cannot be fully oxidized into carbon dioxide and water, which normally happens with fat. Instead it is oxidized inefficiently(reduced carbons still exist in the carbon molecule), then rendered useless and is excreted. These are often called "ketone bodies" hence the name ketosis.
Therefore you do not fully extract the amount of potential energy out of your fat by going through ketosis. For those of us who simply want the fat out of our system, it works perfectly. It's not like we want to hold on to our fat fuel stores as long as possible through extremely efficient use of them, this is the whole point of ketogenic diets.
as far as ingestion of fats. Well when you go on a low carb diet, you generally are not eating as much as 1500 to 2000 calories(depending on how many carbs you normally eat).
If you don't replace that energy with some other source, in this case, only protein or fat. Then you go into a very bad starvation mode where you tend to break down muscle(protein) as energy as well.
So you must replace the energy with both slightly higher protein(to combat protein taken from gluconeogenesis as well), and fat. Fortunately for many of us, we eat enough protein to kind of omitt eating more protein, although this is a very hot topic.
Fat on the other hand. Is a great substitute for many reasons.
1. the lack of fat signals another form of starvation where the body tends to create fat stores.
2. certain lipids can actually liberate fat stores or directly signal adipose tissue that they need not store fat.
3. leptin mainly works in long term energy homeostasis, a lack of leptin or a insensitivty to leptin(receptor) are what cause extreme obesity. As far as leptin's role with the ingestion of fats. Leptin is secreted by happy fat cells.
If you are not consuming enough calories, ghrelin tends to be released, and leptin tends to be inhibited, tells you to eat more to store fat. We want to counter act this effect.
4. The body always gears towards homeostasis. If you require so and so calories per day to function, and it used to come from an anaerobic pathway. And now you consume fat and it must come from an aerobic pathway, you better damn well be sure that mitochondria are going to be signalled to start taking up fat for burning or youre going to die.
Energy is always used in this precedence:
I'm sure there are more reasons which i can't really think of right now.
Keep in mind you don't want to go NUTs on the fat, but you do need quite a bit. If you were previously consuming 300 grams carbs a day, then you need to replace that with about 130 g of fat. which spread of 6 meals can be 21 grams or so.
Yes fat can be stored without insulin, but we're using it at a much faster pace and we are not consuming tubs of lard here. There IS even an article i've read that supports that there are pathways for carbohydrates to be stored as fat in the absense of insulin as well.
if you require that citation i can look it up later.