It is already happened I believe.
http://reprap.org/
Also the results are less interesting, usually...
Being able to print a motor would remove the need to buy motors.
Not every part needs to be "printed". Motor laminations can be CNC cut or laser cut from sheet stock. Then you stack them, bolt them together and add windings. For PM machines you slide some magnets into slots on the rotor. Done. This is fairly easy for hobbyists to do given a good design.
The idea can be applied outside 3d-printers as well. Here is a CNC machine you can CNC (including motion/drivetrain) https://github.com/fellesverkstedet/fabricatable-machines/bl...
One of my personal interests is in making it possible to replicate an full makerspace/fablab by visiting an existing one. Bootstrap one or two digital fabrication machine by borrowing access, then build everything else with those in your own lab. This benefits from high degree of self- or co-replicability.