My serious answer, not an argument: Use d-feet to inspect what’s available on the system and session buses. That’ll show the benefit of introspection and a common serialization mechanism.
About the security comments: Some access control mechanisms aren’t just allow/deny, and many need more than socket permissions. Those benefit from DBus policies, and PolicyKit integration helps for more complex needs. You can always DIY it, that’s Linux/FOSS life, but these are great tools to have in your toolbox. I’ll avoid credential passing via sockets whenever I can and have something else do it.