I don't think the argument is whether or not any extracts beside pilsen or light are ingredients worth using or not. It seemed the intent of the question was wondering why some (I don't feel comfortable saying
most) extract brewers use only pilsen or light extracts in formulating their recipes. I doubt anyone would argue whether or not you could make a good beer with amber or dark extracts. Of course you can. It's more of a matter of control.
While it is true that any pilsen or light extract you buy can be made of a combination of either different strains of the same type of base malt or base malts in addition to some combination of other malts (dextrine malt, caramels, viennas, munichs, etc.),
the intent of using light extract is to mimic as closely as possible the recipe formulation that all-grain and pro brewers use - a large percentage of a base malt (or combination of base malts) and, most of the time, a small percentage combination of specialty malts to control the malt character, color, body, head retention, etc. for the style of the beer being crafted.
I would never discourage someone from building an extract recipe around a darker extract. I would, however, encourage them to think about whether or not they are interested in controlling the aforementioned factors in their brewing process. I am an unapologetic micro-manager in my brewery. I have the feeling quite a few others here are the same.