I think the idea is that if you want to have such a large portion of your realm in a message group, then you might as well just talk to the whole realm.
What I'd rather see implemented is a dynamic message groups system. It was originally proposed on the wiki as Councils:
http://wiki.battlemaster.org/wiki/CouncilsThe idea is that just about every realm has certain groups of people that need to communicate, but have no easy way to do so. Automatically generated councils such as "All Lords/Stewards", "Ruler and all Ambassadors/diplomats", "General, marshals, and vice marshals", etc. would greatly aid in the running of a realm. Trying to manually maintain message groups as a ruler is a real PITA, as you often don't even have all the information you need to keep them up to date. (Who are the vice marshals, who are the stewards, etc.)