I don't have to be sold in the webpage but many others might, I know that plastics are incredibly useful.  First of all they dont have to be made from oil - bioplastic from plant oil, and even biofiberglass - using plant fibers to strengthen the plastic are totally natural and sustainable.  Second the energy footprint for making useful things out of plastic is far better than say, melting and casting and machining metal.  Plastic can be injection molded directly into complex shapes very difficult with metal with no waste and low energy.  Third plastic is potentially much more useful at a lower group technology level, although it doesnt do everything that metal can do, metals require either melting scrap or having a raw material somewhere which may be difficult and energy intensive to mine, but bioplastics could be made from industrial grade plant oils to do many of the same things.

The design list however should not just be extruding plastic, both injection molding would be high on my list, as well as a CNC machine.  The NC/CNC machine could mill/lathe blocks of plastic with great precision potentially with less complexity, the metal toolheads will last long on plastic, and it's an alternative for one off custom parts and such.  The waste plastic is easily melted and reused in most plastics.  Likewise such a machine could be used to make a plastic mockup of say, an engine block, which then serves as a master casting mold for foam or wax, to use a more conventional metal casting method.  Other people online have made CNC type equipment like this using things like old dot matrix printer chassis.  Perhaps the 3 in 1 metalshop could somehow be upgraded to numerical control or full CNC as well, the benefits of precision, repeatability and automation are substantial.

Another possibility is using a type of plastic in an ink jet printer head equivalent that shoots blobs of plastic to build up something one layer at a time.  Or CNC control could squirt out a single rapid hardening tube of plastic while controlling the rate of flow and such exactly.  Though not directly related, monster versions of this are being used in Contour Crafting, which lays out a formulated concrete into organic looking shapes at a house/building level scale.

I'm very big on automating and reducing labor whenever possible, even where labor is cheap or available on a volunteer basis that time is often better spent doing something like farming if it's available, or educating.  There seem to be many ways to automate things with a couple of servos, position sensors and the right type of controller software.   It is not any more excessively difficult than designing a farm tractor from scratch anyways IMHO, it just requires bringing together computer programming, hardware interfacing, stepping motors and position sensors which are skills maybe not as often common as mechanical with the home tinkering crowd. I had planned to try and work on things like this when I had time in the future, due to the prohibitively high cost of rapid prototyping equipment and typical CNC machinery being well out of range of the home hobbyist.  I see no reason such things couldn't be built for a couple hundred (total junkyard scrounging and lower performance) to a couple thousand dollars (high performance comparable to commercial machines costing tens of thousands) and many of the projects could share common design philosophy or parts or interfaces with one another.  An NC or CNC laser cutter can use largely the same control logic as a waterjet cutter, "ink jet" plastic printer, or contour crafting machine.  The only real differences are weights, pressures, speed, and scale.

Are there any others with similar interests to this?

Links for your interest:

http://contourcrafting.org/

http://www.reprap.org/bin/view/Main/ItemsMade - self copying 3d printer made from plastic open source.  One printer can print all of the parts necessary to make another printer, as well as making other parts.