If one designs for it, it is possible to build complex products. FarmBot is a good example. It is made with a combination of 3D FDM printed parts, flat metal parts that could be made with a plasma cutter or CNC, open source software and compute provided by small ARM computers, and commodity produced parts like wires, motors and extruded aluminium rails.
Thats how RepRaps are made, and that is how you have to build in this new way. Very few things are solid plastic shapes.
3D FDM printers are great for designing and producing the case that your etched PCB, switches and LED's attach to. It is an ingredient in the recipe.
FDM can be used to make custom metal components through green sandcasting metals.
If more projects were designed like RepRap, or FarmBot, this new industrial revolution could still take shape. Robotic PCB production is another key ingredient that is needed for reality to match up to the hype.