> One can equally aruge that Linux, and even Solaris these days, have
> source available.  However in at least Linux specialised knowledge
> is definitely required to (re)build any significant portion of the
> system from source.

What does "specialized knowledge" mean?  I suspect more people know
how to "rpm -i" an SRPM and "rpm -bc" the spec file than know how to
use the NetBSD build system.

> I know there are some folks, programmers even, who don't even build
> their own kernels, but that's a conscious choice on their part, not
> something explicitly encouraged by the OS developers.

I think you're making this attitude up out of whole cloth, and it's
disturbing that you attribute it to the OS developers.  Availability
of source does not excuse lack of runtime extensibility for an
operating system.