A domain does not convey any expectations whatsoever - not the expectation to protect it, steward it, shepherd it, or anything else.  A role does that because of its purpose or its accountabilities.  A domain is just property - whether you ignore or caretake your property depends on your purpose or what's expected of you...  So it rarely makes any sense to have a role with just a domain, hence GlassFrog coloring it red - one of the benefits of GlassFrog is that you get some Holacracy coaching wisdom built right in (and much more coming soon!).