Not just prototyping
The economics for use in production would seem to be dependent on the cost and ability to recycle the raw material, the volume of production runs and complexity.
Direct deposition/3D printing has the advantage of very little wastage. So if you are using high grade titanium and know that your wastage can be recycled but only as a downgraded material saving about 90% of the amount needed to mill an aerospace turbine housing from a solid block of aerospace grade titanium is very interesting.
As long as you are within material and printer parameters some forms of complexity become almost a non issue - hence the use by jewelers and so on.
The volume of the run is also important - shouldn't be too long before dental work is a large user of 3d printing - quick bespoke crowns and so on . No need for one - off casting.
(anyone know if anyone can print bronze? should bring back that craft at a much lower cost)
Given the range of material one might wish for I suspect bureau printing (eg shapeways has a material list http://www.shapeways.com/materials ) has an interesting future.
But still - me want. fab@home material syringe seems like fun but the reprap seems more sensible ....