The act of going to an all-you-can-eat buffet does not mean you cannot make responsible choices. You might plan ahead so that you are eating nothing else that day, and you might take smaller portions in order to try what you want to try. Splurging once in awhile does not harm you in the long run as long as you otherwise live a healthy lifestyle.
The average person will eat a lot more than if they were individually paying for it. Depending on the buffet you might be eating multiple high or lower quality meals too. The idea of moderation is at odds with the point of a buffet: pay 1 price, try to eat more than what you paid(most likely impossible or unlikely but people try). As you can't usually take leftovers it makes it even dumber to go for moderation because you're paying for it.
A buffet is the ingredient for gluttony. If you are looking for a sure fire way to ruin your diet, by all means head to your neighborhood buffet and eat up. There is a psychological need to overeat when you know there's limited time and money being spent on this one meal, no matter how disciplined you think you are. Eating on such a level increases your salt intake, not to mention caloric intake. This will expand your waistline, putting pressure on your kidneys and raising your blood pressure. It is just not worth it.
There are healthy items offered at all you can eat buffets but we also see the more tempting fattening stuff as well. I think that these aren't very healthy although they are something I would admit to enjoying here and there. To me this is what an all you can eat buffet is for going to here and there, not going to all the time.
All you can eat buffet's can be a great financial perk but stuffing yourself sick is not at all healthy. The body can only process and use so many calories at once; the rest gets stored as fat. So consuming large amounts of food can contribute to weight and health issues.