The postal service should be worried about 3D printing. Once mainstream home usage becomes commonplace, you would think shipping volume could drop substantially as people begin to buy things online that then instantly print without a need for shipping at all.
I had the same thought, but I suspect quality > speed for most people, and they don't print often enough to justify personal investment in a high-end printing hardware. For example, my wife and I like photography and most of the pictures on our walls are ones we've taken, printed on canvas or high quality photo paper. But we don't own a high-end photo printer, nor do we want to do the work of mounting canvas on frames. I've worked with such devices, they're not that expensive, but I just don't see myself buying a wide-format printer any time soon. I'd rather pay someone else to do so a few times a year. By contrast, I have been thinking about getting a new laser printer for everyday use and reading long documents - I don't enjoy reading everything on a screen.
I suspect that many people may end up owning a cheapie 3d printer for small things, but they'll outsource to a local hub when they want more than basic quality. I'm very into the idea of 3d printing, but the simpler, cheaper units are very limited - as evidenced by people at Maker Faires wearing plastic hats that they printed themselves. Home printing doesn't cut it for a lot of things; for example I use quite a few plastic implements in the ktichen but I would be quite hesitant to use home-printed kitchen implements of any kind.
3D printers don't print objects out of nothingness; they require feedstock of some sort. And as the USPS document notes, that's going to need to either be shipped to you or purchased locally.
