More striking arguments are unfortunately point in time statements, about the current capabilities of technology and the market maturity.
I am not saying that the CNC manufacturing will progress by leaps and bounds towards bringing $100K prototyping machines down to $2K in the next few years, but with increased focus on improving the lower end of the spectrum, and pushing its limits, we will actually find new uses for them as well.
I also don't believe that the ordinary person will go about designing their own parts. However, there will be a good market for people to hire designers, or for small time designers to collaborate to improve open source products.
e.g. One of the biggest ways that I see 3D printing and CNC in general as being important in producing parts replacements. How many times have you needed a broken or missing part for the affordable furniture that we buy from IKEA, Walmart, or Target?
As we use 3D printed pieces, we'll have to use materials which will perhaps be weaker, or in some cases unnecessarily strong but expensive. But, it will be fine, because it will do such amazing things we will not care.