3D printing pens "draw" shapes in plastic [and maybe other materials]. Like a much simpler, cheaper, and probably more limited version of a 3D printer, what are uses for survival prep?

Consider those pens a 3D version of the crayon. They are for rapid prototyping and freehand use. It might be a bit difficult (and expensive to create useful survival items.
You might be better off to try and make some equipment with a hot glue pistol. That works fairly well. It does not have the precision of 3D printers though.

It's just another way to create stuff - might be useful, might not - but definitely expensive.

I believe, such a pen is just a fun way to introduce kids to 3D printing. The whole point of 3D printer is to provide precise way of applying bits of plastic into layers, forming a rigid structure with predefined parameters in fully automated way from a draft. It's near to impossible to achieve that manually, even with steady and artful hands you will have to spend tens of hours to produce even a rather small useful thing. Even 3D printer extruding the plastic line at 6" per second spends hours and hours on that. Also anything you can manually print with such a pen will suffer from simplicity. The software, which is slicing your CAD draft prior to sending it layer by layer to the printer is quite sophisticated. It will take years of practice to gain experience of thinking your hand moves well ahead of the hot melting plastic flow.

Max has nailed it down, it's just a miniature glue gun

Slow and best for very small projects, yes. Ever any small projects in your survival prep? Any use for small plastic forms, seams, electrical insulation, leak repair, cracked plastic repair, possible waterproofing, etc.?

Match this with ability to recycyle plastic waste to feed the beast.

Recycling random plastic bits for a 3D printer is not an easy task. The resulting filament must be extremely uniform, otherwise your print layers wouldn't stick. However, for a manual pen that could be amended by constant visual adhesion monitoring and adjustment - no doubt, that will eventually come with practice.

For an abstract survival use... I'd opt for a glue gun or even a soldering iron anyway, as they're way more reliable, especially if you plan to feed them with random plastic garbage and use in survival settings for tough equipment improvisation All you need to improve on a small glue gun and a powerfull iron is to add a temperature regulator and an interchangeable nozzles receptacle, so you can adjust the base material type and the walls/elements thickness as the job might dictate.

In my survival toolbox I have a small propane/butane torch/soldering iron with several knives and heating nozzles of various forms, similar to this one: http://www.ebay.com/itm/10-in-1-Professi...K-/371134778692 Plus a piece of thick steel wire to improvise custom shapes. It helps to work with various plastics, tin, and glass very efficiently - what should cover most of the survival preps tasks you have outlined above and many more of those a pen cannot handle. It's also a fantastic lighter for your emergency fire, even out of wet wood.

A typical 3D pen, on other hand, works with a specific plastic type only (I suspect 3D pen is just a smart way to sell cheap plastic 10-20 times above it's actual price, as it must come from the pen manufacturer and its affiliates only to maintain the warranty), so you cannot use just any plastic with it without serious modifications. Even the same plastic, after the initial melting will behave differently after re-melting. Too high temp for the particular plastic's sweet-spot and it starts bubbling, thus interfering with extrusion; or "aching", making the result brittle or not adhesive, contaminating the nozzle or the extruding mechanism; too low - inadequate adhesion again, uneven layers, cool-down delamination, or clogging the nozzle too. Moisture absorbed in the filament? Foreign solid particles on the filament surface or within? - More problems with the process and with the result. I'm observing all these effects on my table printer daily and it took a lot of time to tune up the software and the setup itself in order to produce really usable, repeatable, sturdy things on each overnight run. A pen does not provide much of adjustments if any at all.

Comment: its early days for this technology and its use.

I'm sure it will vanish as soon as 3D printer get cheaper. In fact, all you need to build a basic one costs around $50 in mass production. The rest is the novelty profit and convenience attachments.

About the only techniques that will tolerate scrap plastic involve sand casting. Thermoset plastics can be shredded and suspended in a low temperature low viscosity to reinforce it. Thermoplastics, assuming that the type of plastic is matched, can simply be melted and poured. Keep in mind that the resulting product will be much weaker then commercially available plastics.

IMHO, the best skills for self reliance involving scrap plastic would be bonding (glues and fasteners), heat forming (bending, not melting), and mechanical forming (files, drills, and saws).