Methinks you are missing the possibilities in large-scale customization (i.e. single item to short production runs for which producing dies/forms isn’t profitable). Also for producing replacement parts for items that are no longer made, but still in use.
Many of the disadvantages you cite are or are being whittled away. In some cases, the properties can be MORE closely controlled than for regular mass production methods.
One very interesting concept is one which combines “positive” (i.e. deposition-based) 3D machining with “subtractive” (i.e. removal of material with standard CNC) in the same device:
http://store.qu-bd.com/category.php?id_category=23
It looks like their price for the combined unit will be on the order of ~$2500.
My company bought a Roland MDX-40 (not normally thought of as a production machine), and it has paid for itself many times over in short-run production of very specialized parts...the QU-BD unit has FAR more capabilities and costs ~1/4 as much as the Roland did when we purchased it.
The US prevailed in WWII by pushing mass production to the limits. We won because we out produced our foes. That capability was extended past the end of the war to create a middle class with access to wonders that previously had only been available to royalty. Television sets, microwave ovens, automobiles and such became available because mass production drove their prices down to a level that John Q. Public could afford. John could afford to buy these things because he had a good job working in industries that obeyed Henry Ford's dictum to pay his employees enough to buy his products.
Enter the 3-D printer, and our economy sinks to "Cottage Industry" where things are made one by one on the kitchen table. I'm assuming that there remains a store somewhere where one goes to buy the plastic and powdered metals to dump in the hopper but where do the "templates" come from? If you buy the electronic code for a toaster there is no "customizing" going on. If you give John Q. a workstation with a copy of Pro-E on it and expect him to generate the toaster design you are expecting an awful lot from today's public education system. John Q. Public's technological base is such that he has yet to change the spark-plug in his ten year old lawnmower and he has not changed the oil!
Do you expect the end user to become an engineer, designing his own toaster? He has to become proficient in 3-D modeling, strength of materials, finite element stress analysis, heat treating and surface coatings, &c. &c. In other words he needs an actual engineering degree to design real functional parts, not just plastic lookalikes.
Which brings up the question of who or where any post production processes are accomplished like heat treating, ion nitrating, electroplating, sputter deposition, plasma iondoping, and vacuum plasmaspraying, to name a few?
I'll stipulate that John might be a bright boy and figure it all out, but where does John work to earn his daily bread once manufacturing has become a cottage industry? Please don't say "Service Industries"! A service economy is based on the premise that we'll all get rich by taking in each others laundry. It hasn't worked yet...
Regards,
GtG
PS Wonder, how's that "cold fusion" thing working out? I haven't heard much lately...