I can see that unlike a 3d-printer, it's impossible to have a CNC in your desktop because of the waste. But I don't buy the "community" argument. Community has given us things like the arduino, a horrible technology obsolete from the very start. I do not think a technology is better just because many people use it.
Perhaps I should have been more clear. The 3D printing community actively trades tips on how to make their printers better. The hobbyists and certain 3D printer companies alike embrace open source hardware. It is this activity within a community that helps them make progress.
Are similar things happening in CNC machining communities? From my vantage point, it doesn't seem like it.
Cnczone is pretty cool as a community, but I do think the reprap community is more helpful by an order of magnitude. Having met many members of both groups in real life, the reprappers are generally more approachable. I get the impression that every CNC build is a custom design "because I can" whereas most reprappers tend to stick to a few common designs and possibly modify them. Modifications that work generally spread back to the community really really fast and improve life for everyone. The #reprap IRC channel on freenode is an absolute goldmine of a resource, people with questions or problems generally get answers within seconds and it's active, fun discussion the rest of the time. Haven't seen anything equivalent in CNC (cnczone comes closest).
I don't think you can compare RPI to Arduino at all. Arduino is certainly not an obsolete technology. There are lots of projects where prototyping with an Arduino and finishing the project with a super cheap AVR chip is the way to go.
