I think it all depends what sort of diet you are on.
If you are just focusing on calories, then it is just number crunching.
If you are dropping carbs from your diet, then a big dose of them will replenish your glycogen stores, causing fluid retention, thirst and apparent weight regain - I can put on 4-5 lbs over night doing this. Then it takes 3 days of strict low carbing to widdle it all away again - that is half a week lost on weight fluctuations, and not knowing if the weight you are 'losing' is fat-weight or water-weight.
If you are very low carbing, you would likely lose being in ketosis with such a meal. Provided you are fat-adapted, you can slide back into ketosis quite quickly, but you are still bouncing your body around and asking it to switch back and forth between glucose-burning and fat-burning, which seems an unnecessary ask.
If you are fully fat adapted, and your insulin production has naturally downgraded, then dumping huge amounts of carbs into the bloodstream will temporarily send your blood glucose sky high. This happens whether you are diabetic or not, since reduced insulin production happens to everyone on a LC diet. Ever had a food hangover? They are not fun. Takes at least 3 days, sometimes a couple of weeks, to readjust back up to full insulin production. During that period, carbs send blood glucose levels higher than normal, causing more damage than normal. It is advised to re-feed carbs after a low carb diet slowly, over time, in increments, to avoid just this damage.
If you are prone to carb cravings, they will kick in, urging you to eat more carbs, and effectively sabotage your diet. If you find carbs addictive, then that makes the cravings worse.
If you are prone to reactive hypoglycaemia (a lot of us are, although few of us have it badly), then the carb cravings will hit badly, and you may experience hypos after the carbs too.
All of the above are reasons why I think that 'treats' and 'controlled breaks' from diets are really just self sabotage. But then, I get carb cravings, reactive hypoglycaemia, don't lose weight easily, and have seen more people fall off diets than I have had hot dinners. Some of those diets have been my own. So I may be biased, or I may be world weary, or I may be a prophet. Your mileage may vary.