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