A couple weeks ago I talking about how snacking may be related to obesity. If you look at the research on snacking, it is pretty unclear. It often shows benefits in some cases (like adolescents) and is linked to obesity in other studies. One issue that has received greater focus is that the overweight/obese may be underreporting food/snack intake and thus skewing the data.
In my opinion, snacking is not bad per se. If a person is hungry and they eat a snack, that's not a negative event. The problem is one a person becomes a "disassociated" eater – in other words, their eating is no longer governed by hunger and satiety.
In this situation, I think snacking can be a Pandora's box, because of the explosion of high-fat/high-sugar snacks and their constant availability. It would be like if a person is an alcoholic and there is a bar on every corner. The bar isn't forcing anyone to drink but its presence sure helps.
A study from a couple of years ago focused on this topic. The researchers found that the time between eating occassions had decreased over the past 30 years. For example, they found that the time between eating occassions has decreased by one hour compared to 30 years ago. So what could be causing this? Are people hungrier then they were 30 years ago?? I don't think so, especially given the fact that people are consuming more calories today.
I would say that this is a case of, again, eating in the absence of hunger. If hunger isn't driving food consumption, they a person may easily consume food more frequently because of snack availability.
Therefore, I would say that snacking is related to obesity but is not a true cause. Snacking when not hungry simply adds extra calories which can obviously lead to weight gain.