"I would love to see a 3-d printer friend explain to me how it's going to build a guitar from scratch."
I don't make guitars but I am guessing you could 3-d print most of a factory to make guitars. All the jigs and fixtures ready for use accurate to a 1/1000th.
What I do know about, and am interested in, is 3-d printing foundry patterns. Its harder to make a foundry pattern than you'd think, not just the obvious stuff like bulk material shrinkage and draft angles, but its really hard to handle differential shrinkage (warping, essentially). Its a complicated fine woodworking skill (assuming you're using wood foundry patterns). So you can't 3-d print a cast iron pan or an engine block, but you can print a pattern to be rammed in a sand mold and then cast some iron in the mold and ta-da a perfectly flat pan or perfectly straight engine block or whatever. This isn't a magic tool that'll make any idiot able to do foundry work, but it does mean that any idiot on the planet would be able to share world class pattern designs.
I would be happy to be able to print some holding fixtures for my metal lathe. You can't make crazy deep hogging cuts with flimsy PLA holding the work to the mill table, but you could do "something" at least.
I think people who expect 3-d printers to be magic star trek replicators are going to be very disappointed, although indirectly via one or two levels of indirection they will probably play a huge role in everything, soon enough.
Its just positive machining. Much as people don't use my negative machining traditional lathe and mill, I don't think joe 6 pack is going to be using a 3d printer on a regular basis, although much like negative machining revolutionized the world behind the scenes, sorta, positive machining will also revolutionize the world, again, behind the scenes sorta.