i know numerous amateur and pro atheletes who have at one time or another done the all out cheat day diet...not sure about the science behind it but these guys ate like a damn pig one day a week and healthy the rest of the time...
one guy in particular was a top ten ranked in the wbc and an amazing boxer...this cat would eat a cartons of ice cream or handfuls of twinkies and mcnuggets in one sitting. The next day he would be in the gym tearing it up and back on the healty path.
i swear by the 90-10 rule...i think some people on this forum exaggerate the effect that a couple crappy meals a week will have on you.
I'm sure whoever you're talking about didn't do it every single week on the same day.
One meal? Fine. Two? Maybe. But a whole day? And handfuls of twinkies? McNuggets?? I guess I just have a hard time seeing the point of intentionally blowing your own diet for crap that doesn't even taste good.
Notice that I said crappy food will have a huge impact if he was cutting by ketosis. If you are eating twinkies at all during the week you're going to take another 3-4 days after that to get your body back into ketosis. You might as well not be dieting at all in that case.
If you aren't in ketosis, I think the biggest risk you have is the mental baggage that goes along with blowing your diet. Most people are barely disciplined to stick to a diet to begin with. Once you start having cheat days, for them, it quickly turns into "I'll get back on the diet, tomorrow." But it turns out that tomorrow there's a pizza party at work. How about the next day? "Well I've already been off it for two days... What's another day, right?"
It's just not the smart thing to do on any level. If you can avoid it, you should.