I'm not sure who this LL cat is, but he sure makes a lot of sense.
Everyone in the world is on a diet. We have misnomered the word to mean a specific nutrition program - usually to lose weight - and usually lacking nutrition. As LL said, these target type diets are usually too extreme and thus unsustainable, and the metabolic changes they cause can wreak havoc on the body when normal or worse eating returns.
This whole thing - training, rest, habits, nutrition and diet - is a lifestyle. Otherwise, it doesn't work. Finding what works for you is the key.
Over the years I tried it all - low carb, no carb, everything and the kitchen sink, too calorically restricted, etc. Now, I know what works for me. It took time, because I was dumb and ignorant along the way. I still am, but I am learning. Now, I maintain a healthy lifestyle and still manage to get some of the good-bad stuff - chicken fingers from Jack in the Box, fried zucchini at the steakhouse, or a nice bottle of wine - without skipping a beat.
Hope that helps.