clarity of concept

Indeed!

I think a lot of people are under the misconception that libdbus-1 is intended to be used by application developers.

Its main purpose is to provide the basic protocol support for usage in actual application libraries, very similar to how XCB provides basic X11 protocol support for toolkits.

And quite some of the application developer facing D-Bus libraries have moved to their own protocol implementations, e.g. to provide better integration with the target stack's capabilities.