The "one molecule away" statement is more or less meaningless: you could reasonably claim that water is "one molecule away" from gold. But cooking fats (not just margarine) do have a similar molecular structure to plastic, as they are both made of hydrocarbon chains. If you've ever had oily food in a plastic bowl, you may have noticed that it's a lot harder to wash than a glass or ceramic bowl.
Yeah, but there is more to margarine (or fat in general) than just the hydrocarbon chains. There are those clever ester head-groups of the fatty acids, and those nasty glycol linkages. You don't find those in plastics. Of course, you could convert margarine (or butter, for that matter) into plastic if you could just polymerize the triglycerides. But that is very different from the triglycerides themselves (plastics tend to have different properties from the units that make them up). Thus, even if margarine was "one molecule away from plastic," it is wrong to use the analogy of a melted cool-whip bowl on bread. You'd first have to degrade the cool whip bowl chemically into much smaller units, at which point, it isn't plastic anymore. It's more like candle wax (or pariffin). Do you want to eat that? Heck, you do it all the time, anyway, so why not? Give it some flavor, and away you go.
BTW, if you want to imagine "white" margarine before it is colored and flavored, think "Crisco." Pretty close to the same stuff.
Lastly, any of this applies to butter just as much as it does to margarine. The difference between butter and margarine* is pretty much the same as the difference between canola oil and peanut oil. From a chemical structure standpoint, they are pretty darn similar, with the difference in the amount of each component, as opposed to the components themselves. This definately leads to different properties, but neither is inherently all that distinct (palm oil is one example of a "plant" oil that IS very different, at least chemically, from the plant oils used in food, and is more similar to animal fat; always an interesting tidbit)
*Aside from the trans fatty acid issue, which, as has been noted, results during the hydrogenation process (darn equilibrium thermodynamics!). The "no trans fat" margarines look to be basically vegetable oil (in all its unsaturated glory, non-hydrogenated) doped with yogurt to give it some texture.