Prototyping in plastic makes sense here on Earth, where we have a robust hydrocarbon economy, but even so there are problems. RepRap needs all sorts of metal components, including wiring for motors, sensors, and control electronics.

The bottom line is that you can't build self replicating machines with just plastic, but you can build them with just metal and ore (metal oxides).

Now, using lots of different recipes may be more...hmm..."comprehensive" than using just one or two recipes, but it's not necessary and whether it's "desirable" by the aliens depends on their goals and mentality. Do they want to exploit every last cubic centimeter of real estate out there? Just because they can, perhaps? Or do they just want to inhabit places which they find pleasant or aesthetically pleasing? The room for speculation is as big as the universe, here.