Oh yes, the rulers can keep in touch via other means. But the guild, and all the communications it allows, builds a team, a sense of community, among the entire nobility of the involved realms. It creates an "us". That's critical.
Why do you think that communication is intentionally restricted to realms? Because it creates the realm as the team. The realm is "us". Everyone not in the realm is "them". The guild allows you to extend that "us" to include the other two realms. It's the same thing that has happened with the SA theocracies. We have created a very large "us". The ease and frequency of communications allows for a sense of identity. That's the beauty of the guild (or religion). Without that, the 'moot would be no more special than any other group of realms that happened to have mutual defense treaties.
