As a participant in a recent effort to draft a definition of open-source hardware, I've been carefully considering the practices that constitute it: allowing commercial reuse of a design, for example, and publishing the original source files (not some intermediate, read-only format). This aligns with my generally pragmatic attitude towards open-source hardware, which I see as one of many possible approaches, appropriate in some circumstances and not others.
What this attitude neglects, however, and what the definition-drafting effort has largely omitted is the moral imperative for open-source hardware. Given the increasingly large role that technology plays in our lives, it's critical for us, as a society, to know what it's doing. The more decisions we entrust to technology, the more its design determines the policies that govern us. Writers about open-source have long argued this about software, but it applies also to hardware. Devices - physical electronic objects - embed policy decisions in their construction, decisions that cannot, unless the device is open-source, be examined and questioned.
Consider, for example, the machines in airports that check your bags for explosives. What, exactly, do they look for? How do they report their findings? What else might explain a chemical they consider evidence of explosives? Answering these questions requires access to the design of the machine, its software and its hardware. Without its "source", you might be denied boarding on a flight, even arrested, because of the behavior of a device you have no possibility to examine.
Electronics, computation, and network connectivity are increasingly embedded into the objects around us. Unless we know how they're designed, we won't have control of what they do - or what they do to us.