I don't think anyone knows yet what 3D printers will really do to building technologies. All our projections are based on using it to replace traditional methods of making known types of objects. That will be the main use in the short term. But 10 or 15 years from now, we will be using it to do things we can't anticipate now. That's when we'll really see the potential.
This has been the case with all really disruptive technologies: steam power, electricity, refrigeration, computers, etc., etc. We look back at what people THOUGHT we would do with the new technology and shake our heads at their lack of imagination. I'm old enough to remember people laughing at the idea of home computers. "Nice toy, but what would you DO with it? Catalog your neckties or something?"
We are only just now beginning to see really new products and processes from CNC cutting machines, and they are considerably older than 3D printers. At first, CNCs just cut the same old parts quicker. Then you started to get slight improvements on old designs, like the "finger" butt joins in some wooden boat kits. And we've all seen building jigs for small boats made from slotted plywood -- bang it together with a rubber mallet to create a stiff structure with no fasteners. So the next step was house-sized structures built entirely the same way, permanent buildings made from nothing but plywood and no fasteners. (I saw one built at a Maker Faire a few years back.)
Now a few designers have finally branched out into truly innovative stuff. I've seen hinges and expansion joints made from plywood via dozens of partial cuts. Planar rotating joints. Interlocking parts. See MAKE | CNC Panel Joinery Notebook. These kinds of things are theoretically possible without a CNC cutter, but would be prohibitively expensive. Combine these techniques with new engineered wood technologies (see Building materials: Wooden skyscrapers | The Economist for more on that) and things get really crazy.
So my guess is that, in 15 years, no one will be using 3D printers to replicate Ronstan blocks or entire hulls. But every shop will have 3D printers doing something we can't imagine doing now and they can't imagine doing without.