Well... it just switches from checking boxes on "contact some foreign generals" to the game doing it for you. The "contact allied military leaders" version, at least. If anything, that would involve more people than checking the boxes would, because most realms tend to have alliances that aren't immediately relevant, and their generals would typically not get checked in.
Do military guilds really involve more people? I don't really remember them doing that.
The game also dissuades using guilds for this function, because the guild titles are tied to characters, not titles. Whoever founds the guild has total say of what happens to it. If he's protested out a month later? Well... gotta start over... Managing the guild to take into account elections/departures of allied (and local) generals, rulers, marshals, and vice-marshals becomes a tedious administrative task.
Guilds can and have been used for this purpose, but I still don't think it's really a great fit.
