IMHO, rc.conf is a local file.  It shouldn't even be included in the
distribution - instead, there should be a sample file.  When an
upgrade happens, rc.conf shouldn't be upgraded.  Your idea of having
daemons off by default in general makes perfect sense, and is quite
consistent with this approach.  Having rc.conf include rc.conf.local
is also consistent with this approach, but that should be a per-site
decision, not a decision that comes from NetBSD.

As far as documenting what can be configured by rc.conf, why not have
a man page for it?   The fact that the (not distributed) /etc/exports
doesn't have comments in it telling you how to make a new entry
doesn't seem to detract from its usefulness.

			       _MelloN_