That's the whole point of all of this mess: {,k}dbus
Neither an init system nor vertical integration are the goal. The one consistent thing in all of the "systemd mess" is to leave the actual implementation officially a moving target where the trditional .so based library APIs are either hidden and undocumented or they are left out entirely. This forces you to use an IPC mechanism (dbus/kdbus) instead of simply linking to the functions you need and calling them directly. Forcing data to be serialized/unserialized so it can be sent over IPC is not nearly as efficient as calling a dynamically loaded local function. The systemd people love fast thing ("boot time!", etc), so why would they require this slow IPC everywhere?
*** if you never need to link to a library to use it, you can "link" to and distribute GPL code without being bound by the GPL. Poettering's cabal and systemd is an attempt to enable a new form of "tivoization" ***
If you are technically only "using" a library (no linking, no modifications to the library), you have not "infected" your proprietary code with the GPL. It's slower, but computers got fast enough that it doesn't really matter.
The nasty part is that by forcing arbitrary incompatable interface with systemd, to run stuff like GNOME you have to provide the key dbus features even if you don't use systemd. The end-run around the GPL still works with uselessd or any other "systemd replacement".
Unfortunatley, Lennart's cabal has everybody discussing technical features so this obvious goal isn't even addressed.