Mindlessly snacking is bad for a number of reasons, but the biggest one is that it keeps you from actually feeling full. In this video from Discovery News, Trace Dominguez explains how food memories affect humans' feeling of fullness from meals.
Basically, if you think about your food as you're eating it, your brain creates a food memory. That food memory is what leaves you feeling fuller, longer. Absent-minded eating while doing other activities, doesn't allow for those memories to be created. Thus, more snacking leads to more eating — talk about a vicious circle.
Dominguez cites a recent study in which two groups of people were offered cookies. Those who were asked to think about the cookies' texture and flavor ate fewer than those who were eating while reading or watching TV. And when offered more, the television group ate more than the thoughtful group.
So instead of trying to follow through with rigid diets, perhaps the best way to consume less is simply to think about what you're eating, while you're eating it.