There are serious 3D printers by the way that just don't deal in two types of plastic, and other ways of rapid prototyping besides traditional mills, lathes, etc.
You probably meant to respond to the guy that who was talking about plastic 3D printers. I wasn't talking about those, We can use sand and sunlight, the most abundant and cheapest feedstocks we have to print glass. There are printers for printing metal, building materials, human tissues, etc. I get that.
Traditional mill, and lathes, use a subtractive manufacturing process - you take a piece of lumber for instance and remove what you don't want. 3D printers create products using an additive process so in theory they use less material. They should be more efficient and cheaper than the traditional methods eventually.